Playing an MMO will take a lot of time away from your other games, but not so much that you can't still enjoy them. I've played pokemon semi-competitively and WoW almost constantly for the last year and haven't really missed out on any other games that I've wanted to play.
If you want to play an MMO, I highly recommend WoW. It's accessible, popular, and stable. It's streamlined for somewhat casual gamers while also having opportunities for hard core super nerds to poop-sock their way through more difficult content. Here are a few reasons I like WoW better than other MMOs I've played.
Leveling is much quicker and more diverse than a lot of MMOs. If you're crazy you can hit max level in less than a week, if you're committed but not crazy it will take you a few weeks, and if you're mega casual (less than an hour a day on average) it'll probably take a month or two. Additionally (and contrary to what Vulcan said), there are multiple ways to level that are all viable. You can level entirely by doing dungeons with groups of friends and/or strangers, or you can level entirely by fighting other players in battlegrounds, or you can level entirely by questing and killing random enemies. Of course most players do all of the above, just sayin', plenty of options to keep you from getting bored.
Casual. This game is not for people who poop-sock and power game meticulously. Sure, it's an MMO with some difficult content so that element will always be there, but ultimately it's aimed at regular folks. If you mess up your talents while leveling (get one point to spend per level to specialize in different things), it only takes a few minutes and a few gold to fix it. Some games either lack this option entirely or force you to pay huge amounts of in game currency.
Finding groups for dungeons can now be done automatically and almost instantly, this is a welcome change over most MMOs (including the WoW of yesteryear), which require sometimes hours of planning to form a group that might not work out anyway. Of course if you want to design a group a specific way the option to do so the old fashioned way is still there, but the auto option is VERY nice, especially while leveling and when first hitting max level.
The game rarely punishes you. If you die you don't lose experience (lol FFXI), you just become a ghost and run back to your body to resurrect. If you mess up a quest you can start it over.
Quests are simple and the story is available for players who want to know about the lore, but also easily ignorable for players just interested in killing dragons and giants.
Gearing up is easy, any player can get a reasonably strong character regardless of skill level. Only the top levels of raiding are at all exclusive. If you want to explore the most difficult dungeons you have to actually try and be moderately competent, but you don't need to invest huge amounts of time gearing up or perfecting obscure strategies.
Combat is intuitive and simple. No class is difficult to master, and no boss mechanics are frustratingly hard. High level content is about coordination, strategy, and execution.
The classes are balanced. You can be almost any specialization of any class and be useful to almost any given group of players for high level content. You don't have to worry so much about what type of character you make, just choose something that sounds cool and you can't go wrong. No class has a truly hard time leveling or holds back raids. The only balance problem right now that isn't a minor issue is in very high level Arena PVP. If you aren't planning on playing competitively (IE WoW pvp tournaments) then you shouldn't be worried about it. Any class can be competitive in casual pvp if you're a decent player.
The bottom line- Blizzard has done a good job inventing and continuously reinventing an MMO that anyway can pick up and play, but won't bore the hard core community. If you want to play an MMO I highly recommend it. There's a new expansion set to release late this year that is completely redoing almost the entire map. If you pick up the game soon you could probably get a character to max level and clear every current max level raid by the time the expansion came out. Alternatively you can wait until the expansion and level up in a greatly refined world.
Sorry for the long-winded response. If you have any questions feel free to ask!
