Originally Posted by Blizzard Entertainment
Our intent is that most achievement rewards (mounts etc.) should be at the account level. I can't promise that because it's not in and working yet, but that's the intent.
There are probably going to be enough account-level achievement questions that we should put a blog together to explain it all. We'll see if we can find time to do that.
We aren't planning (for now) to have any character only achievements, but there will probably be some account only achievements. These would be things like level a Horde and Alliance character to 90, which wouldn't make sense for one character. Technically, an account-only achievement is different from account-wide, hence the "most." Maybe it's too technical a point to bother with though.