It is probably hideously presumptious to make statements about World of Warcraft on the basis of fourteen days' experience (and three characters none of which has yet passed level 11) but that's exactly what I am about to do. As always: IMHO YMMV. If your mileage does vary significantly, please tell us about it in the comments (or on your own blog and comment the URL).
I've been playing WoW for a fortnight now, having discovered a free-trial offer CD in the local computer store, and have noticed a few instructive similarities and differences between it and Second Life. First, "playing" is the right word to use. Anyone who is uncertain of whether to call SL a "game" should try WoW, the difference will be instantly clear. WoW is a game, people enter it in order to play a predefined set of actions according to a predetermined set of rules in a packaged environment with readymade avatars. SL is a world, people can do anything and everything in it, there's even a near-equivalent of WoW available inworld.
The graphics of WoW are (shall we say) disappointing to eyes accustomed to the glory of SL. Imagine viewing SL scaled down to 32k colours on a screen 100 by 150 pixels, that is roughly how WoW feels to me. It's low-rez and low on physical detail, the avatar mesh in particular is extremely crude compared to SL's.
The avatars are only slightly modifyable: you can select between a dozen skin colours, hair styles and colours, and facial tattoos/makeup — but that's it. The body shape is fixed, and laughably unrealistic. I've made three characters, all female because I simply could not stand to look at myself in one of the male bodies. You spend just as much time looking at your own back in WoW as you do in SL, and I just couldn't take those shapes seriously as "me."
I've spoken before about the standards of politeness, helpfulness and generosity that exist in SL. These are unknown in WoW. Newbies are treated as scum, requests for help or information are either ignored or trigger floods of invective. I think this is because of the competitive and combative nature of the game, my belief is that people feel that helping others would disadvantage them (though in point of fact there is no race to be won, and no shortage of monsters to slay or treasure to win). Updated: age is another factor in this, there is no "teen grid" in WoW. The battlefields are full of children, who act with all the subtlety and fineness of discrimination that one would expect.
There is no conversation in WoW. Starting a sentence with "hello" is socially unacceptable, people react as though you were wasting their time with worthless crap. It's an amusing paradox that I felt that there was more conversation in Dragon Age: Origins (which is a single-player CD-based offline game) than in WoW (which is online with other live humans). People don't talk in WoW. They offer to sell or buy, they ask for volunteers to join them in slaughtering this or that group of monsters, but that's it. Many people don't even bother to say "ty" when you heal or bless them.
Executive summary: It's different from, and in many ways it is inferior to, the visually and socially glorious world that is Second Life; but WoW has an attraction and an appeal of its own. I'm hooked and I will be going back — to both.