Ahh ok, I see my mistake. When you create new criteria you are given about 10 or so criteria boxes for adding new ones.
I was thinking that these were tied together, not that it would create separate ones.
So basically I could do a criteria like:
posts > 100 AND DaysRegistered <= 30
Then add an achievement for "Posting Machine!!"
Or something. Basically being able to have more conditions per criteria.
So, since you can't do that now, I'm going to post it as a suggestion in another thread. =)