Are there any achievements related to donating # of items?? If not, I think that would be pretty neat - a good way to keep the donation corner active as well, for newer users!

Aside from clogging the donation corner filled with junk items, I can't really see what it would be good for.

I like this idea. Sometimes the donation center has almost nothing in it. Right now there's just 68 items.

Clogging doesn't sound like an uncommonly bad thing to me. The Achievement won't go away or wear out so once all the tiers are done there won't be a lot of mass-dumping anyways. Some users chuck everything they own into the DC before they leave. I confess it amuses me silly to imagine that they will get unintended Achievements for this.

I know that staff has on more than one occasion cleared out the Pawn Shop. If clogging becomes a big deal they could do the same.

IMO nothing that can be Recycled or Fragmentized is junk.