Things that break your game's skill cap on attacks are probably a dumb idea. Things that can get you an entire combat's worth of attacks at double your game's skill cap (Oh hi, focus items!) are a super dumb idea.
Be upfront with people about what you're going to let them do with thaumaturgy. The ability to do anything with essentially one skill, especially where one of the explicitly listed uses is 'Do impossible things', has the potential to get way out of hand. :-)
If your group has a 'that guy', maybe encourage him to not be a wizard.
That's one issue Bedurndurn, out group is going to be a large-scale online chat with a high number of players and multiple GMs, we haven't opened yet and we already have 30-something registered users, that number is only going to grow. I'm inclined to trust everyone but in a group that big you can't keep an eye out for 'that guy' as easily as you can in tabletop.
Thanks for the advice so far everyone, if anyone has more to add please do.
I found that discussion about shifters interesting, their disadvantage of having to shift form really is a nice inbuilt limitation.
Evocation with Focus Items is still one of the biggest worries, yeah. But the more time passes, the more I'm inclined to let things go as they are in the books in most cases and watch how things develop for the first few weeks of the game.