The four big ones I'd like to see are
150 mounts is a gimme at this point for a lot of people, and it will be even more so with the plethora of mounts available shortly after release in MoP. 200 mounts will still take a bit of work on release to get for people with a lot of mounts now, but you should see this on most servers 90 days into MoP.
Now 250 and 500k HKs probably aren't going to be all that popular but hear me out. These are called achievements for a reason, it gives us something to work toward. There is nothing wrong at all with not getting achievements the day they are released and it should be motivation for any collector to keep pushing forward.
What are some achievements that you'd like to see?