I've noticed another great and I think significant difference between Second Life and World of Warcraft: Connectivity and the context of my identity.
SL is a single, continuous, large world which is distributed in small pieces across many hundreds of servers. Each server holds a unique portion of the world, and all avatars can move freely between all servers. IM communication works across servers, you can speak to your friends regardless of where you or they are.
WoW is more like a set of parallel universes: it is a relatively small world which is duplicated identically on many hundreds of servers. Each shard contains exactly the same physical world and the same storylines, with the same monsters lurking in the same places and the same NPCs giving out the same quests. An avatar is restricted forever to one single shard, and can only interact with avatars who reside there. ("Forever" is a relative term: you can apply to have your avatar moved permanently to another server, but it's a one-way process that takes a week and costs a month's subscription fees.)
Because the WoW servers are discrete (unconnected), it suffers from the same disadvantage as OpenSim: your character and your identity exist only on a single server, and nothing prevents a malicious person from creating an identically-named character on any other server. This has obvious consequences for your reputation: how would you explain that the person creating havoc under your name in a sim you've never visited is not you? Why should you need to explain this?
RL deals with the problem of non-unique names by creating artificial identifiers (passports, drivers' licenses and the like). Websites like Avatars Online represent an attempt to achieve this by letting people declare all their identities in one location. Perhaps OpenSim needs a similar kind of central registry?
This isolation of avatars from each other has always struck me as the great weakness of OpenSim. Why would I leave SL, where my friends are, where (as a builder or musician or maker of clothing) my possible audience and visitors and customers are — to move to a place which has none of those? I don't believe that people enter online virtual worlds in order to be alone; certainly the majority doesn't, and the evidence of SL is that those who remain alone usually drop out fairly quickly.
That is different in WoW because the game environment keeps you busy: at lower levels, most races and classes don't need other players to interact with. You can't get far into the game without joining a guild and taking part in group battles, but there is no other need or reason for sociability there. And indeed there is no sociability to be had.
In other news I picked up a WoW special-issue gaming magazine in the airport on the way to Canada, and read some more background about the various races and classes, and have made yet another character: a human paladin. From the description, they can play the healing role that appealed to me, but also have combat abilities that the priests are lacking. I'm currently trying them both, and will decide soon which character will be my "prime."