The first thing that players run into is the simple fact that roleplaying is time spent in the game without being time spent playing the game. Unless you're roleplaying through content, absolutely no game progress takes place, and if you are roleplaying through content, you're probably going a lot more slowly than you would be otherwise.
Does this matter? It does if you want to keep pace with anyone else in the world, since it means that any given play session is made up of less actual play. If you log in for a three-hour session and spend half of that roleplaying, you're going to be lagging behind anyone who spends three straight hours playing. Leveling, raiding -- whatever you like to do, you're going to have less time for it if you spend more of your time in the middle of a lengthy conversation.
On the other hand, if you enjoy roleplaying, advancing that whole sprawling plot is a big part of the reason you keep playing anyhow. Shifting yourself over to a footing of just playing the game removes your main incentive to keep leveling in the first place. You have to strike a balance between gameplay and roleplay, and it's not one that comes naturally since you're missing out on one every time you choose the other.
But why does it matter, right? If you're in it for the roleplaying, you don't really need to care about frivolity like levels, right? Except that you do.
The game does, in fact, matter
Roleplaying will stretch the rules of the game quite frequently. There's no way to model the fact that my Jedi Sentinel in Star Wars: The Old Republic is a former Sith apprentice, so she started at level 1 even though she should logically have a lot more experience. But it's a lot more plausible because she's of a specific class. If I had made her a Smuggler, the whole conceit wouldn't work at all. I'm stretching the rules a bit, but I'm still aware of them.
Let me put it much more simply: If the game didn't matter at all, we'd all be in Second Life or points related, games classified as games only because of a lack of any other useful descriptions. As much as the game might stymie certain amounts of roleplaying, it encourages other avenues and gives you a setting and constraints that you can play within.
And it also gives you mechanics that you either want to play or at least don't find screamingly unpleasant. I've seen a few people try to stick it out in games they dislike; it didn't turn out well. Roleplaying alone will not keep you in a game you loathe.
Despite this, we don't want the game to be the sole arbiter of our characters. Even if we set aside issues like the fact that there are roughly a million people who can lay claim to fighting the big bads in World of Warcraft, there's the simple fact that most games assume an ageless and faceless character when quests are written. The game won't acknowledge that your Elezen in Final Fantasy XIV has lived in Gridania his whole life and joined the Wood Wailers eight years ago; you still have to work your way up and go through the content. But even if you ignore some of the text, making your character into a Wailer is only possible because the game lays a framework for what a Wood Wailer actually is.
Passive and active roleplaying
Part of making up this split is a simple question of time management, and everyone is going to land at a different note on that scale. Another part is the matter of passive roleplaying, and this is something that's present in every game but particularly pronounced in a few.
We tend to think of roleplaying as a very active hobby because it needs to be. You need to make the time and effort to establish your character. But there are also a lot of small choices that you make about your character in the game that contribute to a picture of who he or she is, and part of managing your roleplaying is both cultivating this passive roleplaying correctly and noticing it in others.
What areas are you questing or farming in? What equipment do you wear (assuming you have some control over the visuals)? What sort of mount do you ride? What sort of activities does your character take part in on a regular basis? What title do you display? What's your character's specialty in games that have a talent system?
From a gameplay standpoint, there's no reason for my character in Final Fantasy XIV to do leves in Gridania as opposed to Ul'dah. But in-character, if she's trying to curry favor with the Gridanian authorities, it makes sense for her to be there. If someone runs into her there, it'll make sense for a bit of impromptu roleplaying or even to just put the picture in another player's head that you're hanging out there. Similarly, the fact that my Rogue in RIFT has points put into the Bard tree is just sound strategy for a tank... but it also says something very important about her history.
Star Wars: The Old Republic has a certain unique support of passive roleplaying via the dialogue trees, which forces players to think about their characters even when not actively roleplaying and gives them a sense of individuality when grouping up. This isn't the same as a full session, and it's not something that every game has, but the fact that it exists is just another boost for roleplaying in a passive fashion as well as an active one.
There's always going to be a split between what you do in-game and what you do in roleplaying. But a little extra effort to make those facts line up can go a long way toward making your overall playtime more productive and fun.
Want to offer feedback? The comments are below, and of course you can always email me at firstname.lastname@example.org. Next week, I want to give praise to the often under-used tool of the stereotype.
Every Friday, Eliot Lefebvre fills a column up with excellent advice on investing money, writing award-winning novels, and being elected to public office. Then he removes all of that, and you're left with Storyboard, which focuses on roleplaying in MMOs. It won't help you get elected, but it will help you pretend you did.