Haven't had a whole lot of time to bang on the account, but here are the important points from the little time I've spent with the beta:
1) the game runs via a browser plugin, and as far as I can remember, it's exactly the same as if you were playing on the PC. Graphics seem to be better than what I remember of q3a, and the game runs suprisingly smoothly considering the environment it's in.
2) the main difference is scripts/aliases... not sure if it'd be possible to load up my old complex key bindings
3) when you create a new account, you go through a "tutorial" with Crash (the easy-kill chick from the first level), but there's a twist. As you keep fragging the bot, it subtly notches the difficulty up to figure out where you're at skillwise. I didn't notice this until the bot suddenly side-strafed a rocket that should have gone straight into her face, and she ended up just chainsawing my lead down in pretty short order. :(
4) right now, it's kind of hard to find matches because there aren't many folks in the beta. It looks like when everything is live (or when more invites go out), though, getting into a decent game with folks around your skill level should be a piece of cake.
And yes, there does appear to be a way to send out invites to others. No, I don't seem to have any at the moment.
Personally, I think this is going to fill the "I just want to shoot something in the face" niche quite nicely.
Update: Found the invites \o/ and got 3 left. Drop me an email or IM if you want one.