I would probably recommend World of Warcraft for somebody fresh to MMOs. It's not very time demanding compared to the other MMOs out on the market, and it can keep you entertained for a few months.
Before this expansion, I must admit, I did enjoy WoW alot more.
It was rich in lore, the gameplay was good; and it was very atmospheric.
But since TBC has come out.. we've gone from a traditional fantasy setting to a space age techno-land grindfest, with tentacle men who look like they've come from a cheesy episode or Dr.Who.
In my opinion, this game has lost it's originality and roots.
If you fancy spending hours upon hours reaching the level cap by quests, which in essense are all the same (Kill X amount of Y, Collect X amount from Y monster, Kill X amount of Y, etc, etc, etc) ; then sure, go ahead and buy.
When you reach the level cap (level 70, it takes a few months to casually reach this level) you will start to realise that it's just the same as the grind to level, in essence: but now you grind for gear.
Get killed 249872387 times in the very limited amount of Player Versus Player battlegrounds before you can buy X item.
Do that over and over til you have a full set.
Or, do 3209734287 quests before you have enough reputation to reap the rewards for X item.
If that doesn't please you, then raid the same instances several times for several hours each time, the encounters not differing in the slightest for a 10% chance for X item, which Y player might need too.
Now, these things can be fun, yes; if you do them a few times.
But when you experience them 10,20,30,100 times, they do get quite tedious (and trust me, you'll need to do them 10,20,30,100 times to progress further into endgame).
Aside from that; the community (at times) can be quite terrible; and just about all the content from the original WoW is rendered useless as the items gained from there suck in comparison to the newer items.
Again; I'm not trying to stop anyone from buying WoW, as it can be a good game at times; but just realise how simple the game really is, and how 1 directional it is in most aspects.
There aren't any hugely sucessful MMOs released at this moment in time that can compete with WoW's popularity, but by no means, don't be swayed by the supposedly "10 million" subscribers.