There are two main issues with the present design of the Feral Druid class. The first, and more serious, involves its desirability for raids. The second is the lack of choice in its talent options. I will discuss the desirability problem first.
Overall, I consider Feral Druid to be a viable class for raiding. In sustained single target damage it performs competitively, and this is at least a relevant capability on most raid encounters. However, while it's a viable class for this reason, it's certainly one of the least desired classes to have.
The main problem with the Feral Druid is that it's a highly specialized class that isn't even exceptional at its specialty. Feral Druids are good at two things: sustained single target damage and sustained 2-target cleave. At the former, they are strong but not exceptional. Feral Druids perform sustained single target damage at about the level of Shadow Priests, Ret Paladins, Rogues, and others -- below Warlocks and Mages. At the latter, Feral is merely above average; Feral Druid 2-target cleave is decent, but no match for the likes of Monk, Warrior, or even Warlock or Rogue.
There would be nothing wrong with being merely "good" at these forms of damage dealing if the class were also competitive in other scenarios. But it's not. For one, Feral Druids have no viable AoE -- something about which we've likely seen many complaints. Less obviously, the class is also weak against short-lived targets.
That the Feral spec lacks viable AoE is certainly obvious to anyone looking at damage statistics from Highmaul, though this deficiency is perhaps less crippling than such statistics may suggest. Many fights have an AoE component, but usually it is not necessary to have the whole raid assist with this AoE role -- whether because only a small group can help with the AoE (Kargath), because the AoE targets have low priority (Mar'gok or Darmac), or because the AoE targets have little health (Ko'ragh). That said, there are fights (like Tectus) where it is of the utmost importance that every member of the raid do as much AoE damage as possible, and there the lack of viable AoE can be a real handicap. Overall, it's not essential (at least for raiding) that Feral Druids be able to put out competitive AoE damage, but the absence of this capability certainly reduces the versatility of the class.
The more insidious deficiency of the class is its weakness against short-lived targets. Examples of such targets include Ko'ragh's mind controls, Mar'gok's Aberrations, Darmac's spears, Kromog's pillars, or various opponents in the Blast Furnace or Operator Thogar encounters. This deficiency essentially arises from the Feral Druid's reliance on the periodic damage from Rip and Rake, which often cannot tick fully against short-lived targets. I should clarify, however: Feral Druids do have the ability to burst down a single priority target by pooling resources for Ferocious Bite, but such preparation is expensive and typically costs substantially more damage to other targets than what it gains on the priority target. So for example, Feral Druids must give up overall damage in order to deal competitive damage to the Aberrations in the Mar'gok encounter. In contrast, there are other classes that actually gain damage from focusing a short-lived target: Shadow Priest via Shadow Word: Death and Twist of Fate; Monk via Touch of Death; Marksmanship Hunter via Kill Shot; and more. While the weakness of the Feral spec against short-lived targets is not quite as obvious to the casual observer as its weakness at AoE, it's likely more important. Raids are rife with short-lived high priority targets -- as the previous examples indicate -- and it is a great blow to the class's viability in raids that it cannot cheaply and effectively prioritize them.
As I see it, there are two approaches to the versatility problem. One option is to embrace the specialization of the class and to simply make Feral Druids exceptional at what they can do. This was the design of the class in Siege of Orgrimmar: Feral had most of the same weaknesses it has now, but was carried by its exceptional single target sustained damage. In the present case, the options would be to raise Feral Druid single target damage to at least the Warlock/Mage level, or to improve Feral Druid 2-target cleave (likely by buffing DoTs). That path is simple enough. The alternative option would be to make the class more versatile, and I wish to discuss options for accomplishing this.
To address the spec's weakness against short-lived targets, one minimalistic attempt at a solution might be to change the Glyph of Savage Roar to trigger when you Rake or Shred a target above 80% health (instead of triggering when you use those abilities while Prowling). This would add a benefit for switching to a fresh target, addressing the problem of the cost of such a switch. It would also make it easier to prepare a Ferocious Bite for the target (due to not having to ensure Savage Roar stays up), addressing the problem of the class's effectiveness in such a switch. This change would maintain the flavor of the glyph in that it grants a free Savage Roar when you open on a fresh target, while also removing the weird synergy between this glyph and the Incarnation talent. Note that such a change would amount to a 1-2% DPS loss in a pure single target encounter (depending on duration), so adjustments may need to be made to compensate this. I would emphasize again that this is a very modest solution: in the ideal case where the player is able to maintain Savage Roar via this glyph alone, the overall DPS increase is still only 3-4% over what it would be today (or about 5% after the aforementioned adjustments).
Other solutions might involve making Ferocious Bite refund some resources if the target dies within some duration. This makes a certain amount of sense thematically, but as an entire new mechanic, it's likely outside the scope of any minor patch or hotfix.
Addressing the spec's weakness at AoE is pretty simple, but some care must be taken. First, any increase to Thrash's damage per energy makes it worth using against a single target. I'm not convinced this is a bad thing -- it's interesting to maintain another DoT, especially when it's a major resource dump and you have to worry about whether it'll prevent you from landing Bloodtalons on your next finisher -- but it does reduce the number of Ferocious Bites you cast, and Ferocious Bite is a fun ability to press. Swipe is a bit more straightforward; its damage could be as much as doubled before it overtakes Shred, and note that Swipe is in no danger of overtaking tab Rake for cleave damage: even with Swipe doing double damage, it would take 5 targets before it was worth casting over Rake. However, at this damage level Swipe becomes DPS neutral (or possibly even a gain) relative to Shred for single target damage when there enough other targets, owing to the way Primal Fury triggers if Swipe crits any of its targets, and this may be undesirable (although it's not particularly unusual for a class to have free AoE).
Another comparatively easy option might be to make the Force of Nature treants cast Thrash (at some appropriate tuning) instead of Rake. This spell would then function as a sort of mini-Bladestorm, and such a change could have other benefits too. A minor utility of the Feral spec is that it has powerful cooldown damage -- useful for burst phases in fights, like Mar'gok intermissions -- and putting a large portion of the Feral Druid's AoE damage into the Force of Nature talent would prevent the spec from having both strong AoE and strong cooldown damage. It would also be possible to tune Force of Nature below Incarnation for sustained single target damage to compensate for the added AoE capability. Finally, this would give a niche to the Force of Nature talent and make it an interesting button to press. I would note, however, that there is a strict upper limit to the amount of AoE damage that the treants can contribute. With Incarnation being worth ~11% of our DPS (~8% if the Glyph of Savage Roar interaction were removed per the earlier idea), the treant Thrash must contribute less DPS than this against a single target (even accounting for trinket proc gaming), lest it become the new top single target talent.
This leads us to the other issue with the Feral spec: its lack of talent choice. Feral has two talent rows relevant to damage output. In the level 60 row, Incarnation is the only viable choice. In the level 100 row, Bloodtalons is almost always the best choice, with Lunar Inspiration holding a small niche. Talent options for other classes are often a major contributor to the class's versatility -- for example, the Serenity/chi Explosion (Monk) or Bloodbath/Bladestorm (Warrior) choice offering the ability to specialize for single or multiple target damage -- and improving the Feral Druid's talent selection could be one route toward boosting the versatility of the class in raids.
In the present design, it's probably a good thing that Incarnation is the only viable level 60 talent, because it's the only one that is interesting to use. I was hopeful for Force of Nature in Siege of Orgrimmar, with the gameplay of dumping your treants on temporary stat increases. However, the ability still never felt interesting to use, as trinkets were the only real consideration for how and when to use it. There are ample options for making Force of Nature interesting, but most would involve adding a new mechanic to the ability (some sort of interaction with the resource system, for example). However, one very simple way to both make the ability interesting and give it a clear niche would be to make it deal AoE damage -- to make the treants cast Thrash instead of Rake, as discussed above. As for Soul of the Forest, I don't know how to give it a niche in serious play without a mechanics change, but with a bit of upward tuning (or perhaps none at all, if the Incarnation-Glyph of Savage Roar interaction were removed), it could serve as a viable beginner talent.
The level 100 talent tier is a bit better. Claws of Shirvallah will never be used in raids, but it's a strong PvP talent, which is enough for now. The main objection I have to this talent tier is to how small Lunar Inspiration's niche is. It performs marginally better than Bloodtalons at sustained 2-target cleave, and performs substantially worse in just about every other scenario. My suggestion would be to raise the Moonfire's duration to 20 seconds (from 14) while maintaining its damage per tick. This would make the talent stronger against multiple targets -- especially more than 2 targets -- without substantially strengthening it against a single target. Moreover, this change only makes sense: the ordinary spellcaster Moonfire also lasts 20 seconds.
I hope I have successfully conveyed my objections to the present state of the Feral Druid class. The main problem is that it's a highly specialized class that isn't strong enough at its specialty to justify it. The class needs either to become stronger at its specialty or to become more versatile. I have offered some suggestions toward accomplishing the latter.