Hm, I for one complain only about two things related to the Mogstation:

1. The prices are too high; especially for glamour that goes not to the account but the individual playable characters.

(I'm so glad I got the original Maid- / Butler-costume as well as Eastern Journey set. Having them bound to my account instead of a single character definitely makes playing with alts more fun.)

2. Stuff from the cashshop cannot be sold ingame.

(One of the small things I like about Tera is said feature. If you don't want to spend real money on a costume / mount, you still have a chance to buy it from another player with the ingame-currency)