I've seen the 7.5 expertise other places as well. What's the advantage/disadvantage of having 7.5 instead of 15%?
The same disadvantage as having 0% compared to 7.5%, tbh, you'll miss 7.5% of your hits that you otherwise wouldn't. Expertise scales pretty much linearly from 0 to 15 if you're attacking from the front, as tanks do.
They changed that with MoP and it went a little under the radar. (any guide that says to stop at 7.5% without providing a compelling explanation I would almost recommend ignoring, since they don't know their stuff, but you didn't hear it from me) --- One day dude, I'm just gonna get off the bus, and I'm gonna run in the woods and never come back, and when I come back I'm gonna be the knife master! -The Rev