I want to keep this brief while still keeping to the heart of my argument. The community has recently discussed the relatively low population of rogues comparative to other classes, and the Blizzard posters have acknowledged it. Rogues are less populous than they ought to be statistically, and this trend dates back even to when rogues were at the top of the food chain. A lot of players in the community are putting forth the hypothesis that this is in large part due to the glum feeling of the community towards rogues in their present state -- their feeling weak in PvP and general gameplay after the MoP nerfs. The Blizzard posters have discredited this largely, due to the fact that though some rogues have quit during MoP, the trend by and large dates back before that. I want to present a different perspective on why the population may me less than stellar.
For a bit of background, I've been playing rogues since mid BC. I'm no Neilyo, but I'm fairly well-versed enough with the class after a few solid years of experience with them. I'm one of the rogues who quit during MoP (because in honesty I found my other characters to simply feel more fun after the recent changes), but I recently started up a new rogue alt to return to a class I love in between farming conquest points.
Leveling up in the earlier levels, I'm reliving something I've altogether forgotten about rogues up until now: they're altogether dull at low levels. While many other classes get interesting class-defining abilities--or at least things which lend their gameplay some sentiment of variability--at low levels, rogues do not. For a good chunk of the early leveling experience, rogues feel like nothing more than single-button mashers, doing a whole lot of nothing. Even if they are tuned to handle content at these levels, they feel as though the content is posing nothing remotely resembling compelling action-RPG style.
It's not entirely so that rogues are worse off at early levels in this respect, because other classes seem dull on paper early on as well. But unlike rogues, many other classes have primary mechanics of damage which give the player the sense of something happening. A Frostbolt hitting its target and shaving off a non-negligible amount of its health feels a lot more enticing subconsciously than an amount of Sinister Strikes doing roughly the same damage in the same time. Primarily because rogues have lackluster animations, but mostly because it feels rudimentary. Conjuring bolts of magic up gives the player some sense of a fantasy element behind what they're doing -- something they can hope to develop to more compelling depths. Rogues on the other hand just feel like they're whacking away at poles with wooden swords: it looks and feels just about as compelling as games children play with twigs and branches.
Recently my younger brother -- and then his friend, and another of his friends -- all joined WoW. Like most new players they didn't know what class to choose, and I recommended they all try rogues. At the point I was already high level and it'd been a while since I had to endure being a low-level rogue, so I was surprised when they all found it terrically boring. All but my brother ended up settling on other classes because they felt more fun and (quote) "strong" than rogues. And leveling a rogue again, and remembering back on the other rogues I've leveled, I now remember what they mean when with their sentiments of rogues feeling lackluster for low-level players. It brought back memories from since I've first started playing rogues that, the class just isn't what it ought to be until at least the mid 30s. Unlike any other alts I've leveled, the class just doesn't feel like itself until then, at least -- and even then, it doesn't feel truly compelling until closer to 50.
For the active community at large, this probably doesn't mean anything. Everyone is so used to playing at max level that the early levels are seen as nothing more than a burden to be gotten through as quickly as possible. The contrast between playing their level 90 and playing any low-level character is so large that every low-level character will have this mundane feeling to them. But remembering back to my days with my first rogue -- and comparing my current rogue alt to 2 other alts I just began leveling -- the rogue class does really stand out as being fantastically boring early on.
Again, this doesn't mean anything to a high level. No rogue is going to quit their class because it feels dull at early levels. This does not at all account for any rogues that have been quitting lately. But I believe it does account for the trend of underpopulation in the class dating back the last couple expansions. A good chunk of the playerbase seems to have joined within these last few expansions; and when it comes to choosing the class that will eventually become their main, monotonous gameplay could make a statistically significant dent in the population numbers comparative to other classes that feel more interesting to a new player -- a person new to WoW, or even one looking to play a totally new class is more apt to choose one that feels more compelling than beating a dead horse.
If you humor my hypothesis, the solution is simple, clean and easy. Rogues need to feel more well-defined at lower levels. The suggestion has come up to revamp their animations, and we've gotten the response numerous times that there's only so much you can do given their aesthetic style -- and I agree with that. But I don't think the solution ends there. Rogues can feel epic to new players without necessarily looking epic.
Rogues by and large are defined by three properties of the class: burst, control and (despite recent arguments to the contrary) mobility. Yet: rogues have no real burst (unless they have heirlooms, which no new player will have) until well into the vanilla content; the first good control ability (Cheap Shot) is not received until level 30; and rogues have nothing aiding their mobility until about half as long (Sprint doesn't come until 26, and Shadowstep takes until 60 to get!). The rogue absolutely does not feel like a rogue until a good few weeks to a month into its gameplay (for a new player). While other classes get at least a few class-defining abilities early-on to set the stage for what's to come, rogues get close to nothing, and what they do get is largely the abilities that (may give aid gameplay on paper, but) don't give rogues any more "compelling" a feel. In other words, a rogue with Shadowstep feels a lot more nimble and interesting, even if Shadowstep wouldn't aid low-level gameplay, whereas a rogue with Gouge doesn't feel all that much stronger, even if it does absolutely come in handy early on. To a new player, the matter of their perception of their class's strength can be important.
I think the rogue population would see a small but trending upturn if the levels at which these certain abilities -- the ones that give more feeling to the gameplay, and the ones that hint at abilities and structure to come -- were reduced. Give new players a stronger taste of what the class really is, and I think they'll be more prone to stick with the class, lending rogues a statistical advantage in terms of boasting population.