Who will get the least out of playing a Druid?
The Druid is not a good class for people who want to perform one role and one role only, or whose primary enjoyment of the game arrives in the form of topping damage meters. It is also not a good class for people who are accustomed to having a lot of bank and bag space, as enjoyment of the class' much-vaunted versatility arrives at the cost of having to collect, gem, and enchant multiple gear sets. The expense for this is, as you might expect, considerable. Moreover, possession of a gear set doesn't remove the responsibility for having to learn how to play the corresponding spec effectively; unlike most other classes, each of the Druid's specs requires an entirely different playstyle and role within a group/raid. Consequently, people who can play all four Druid specs fluently are very rare, and it might be accurate to say that the Druid makes a better main than an alt.
Because the Druid is capable of playing each of the game's four roles, you should realistically expect to tank or heal at some point -- or at least get pestered about it. It's fine if you have no desire to do so, but be prepared for the inevitable requests.
Despite the improvements made to them, the animal forms lack the options available to the pure classes they mimic, and there's a roadblock artificially imposed on reaching the options you do have. A Protection Warrior would have to cycle through 3 or 4 bars to grab all of their abilities during a fight; a Bear can fit his/hers on a bit more than 1. This is the result of Blizzard considering Druids holistically rather than as separate classes whenever they shift.
What the hell does that mean?
It means if the Warrior and Druid both have 100 skills (not an accurate number, just work with me here), then the Warrior can put most of them on his/her bars; the only thing standing between him/her and another ability is a stance shift (or talents). The Druid, by contrast, will see 20 abilities available only to Bear, 20 available only to Cat, 10 from talents, 50 available to caster/moonkin/tree, etc. Blizzard still considers these all Druidic abilities, rather than "Bear" or "Cat" abilities, so the fact of the matter is that -- regardless of the form you currently occupy, most of your abilities are not immediately available to you. In order to use all of your helpful or emergency skills, you need to learn how to watch your mana bar (an OOM Druid is an optionless one) and get used to shifting quickly in order to get the skill you need. Thus the mana for shapeshifting is a sort of hidden cost imposed on top of the cost of the skill you want.
That doesn't sound so hot for PvP.
Druid PvP has historically been a feast-or-famine type deal, and it's probably no mistake that the spec that shifts the most (Feral) has been abysmal since day one. Restoration, as you might recall from the screaming matches on the forums in Seasons 3 and 4, did well in 2's during BC, but has gone back to the feast-or-famine model. Balance is your next-best option in arena, but it's a distant second.
You mentioned something concerning canaries. Do tell.
I find patterns in Druid spec changes to be interesting signs of the shifts in wider class/role popularity. Because we can play anything, a larger movement into one role or class often winds up having an effect on what Druids (voluntarily or otherwise) play in raids. As a fairly immediate example, with the rise of the Death Knight, the overhaul to the Protection Warrior tree, and the de-suckaging of tanking in general, players flooded into tank roles -- but within the month after Wrath hit, the Feral spec went from being a 50% share of the Druid population to around 33%. Why? Because someone else was tanking, they had fewer options than we do, and as a result, more of us wind up DPSing or healing.
This doesn't necessarily have anything to do with the viability (or non-viability) of a Druid spec at any given time; indeed, I would argue that Feral was in a pretty good place overall in December 2008, and it's in a pretty good place in July 2009 with the exception of a few concerns related to bear mechanics (more on this in a future article). But the increased availability of tanks/healers/DPS from another class and spec pushes committed Druids to respec and come as something else rather than rerolling or playing an alt. If you see a ton of Druids respeccing to cover a particular role, that usually means there's an absence of such players elsewhere. Food for thought.
In summary -- there are a lot of compelling reasons for playing a Druid, and a lot of equally compelling reasons not to. I think out of all players, people who already have a tank and healer alt at their disposal may get less out of leveling a character with those capabilities, and they will be more prone to feeling the irritations of the Druid's constant need to shift. Druids are also difficult to level initially, but become enormously fun to play in time.
Anyway. Before 3.2 hits, I would like to have a full guide to leveling a Druid up, complete with information and advice on glyphs, enchants, gems, rotations, talents, and more, so watch this space. With the proc bugs on the PTR build that went live the other week, I wasn't able to assemble an accurate list of numbers on Balance DPS, so I'm still keeping an eye on that, and I'll have more information when it becomes available.
Patch 3.2 will bring about a new 5, 10, and 25 man instance to WoW, and usher in a new 40-man battleground called the Isle of Conquest. WoW.com will have you covered every step of the way, from extensive PTR coverage through the official live release. Check out WoW.com's Guide to Patch 3.2 for all the latest!