achievements are very important to me, its hard to describe the satisfaction you get from seeing the achievement pop up. I like the group achievements like killing a boss in a certain way, or ones linked to events. For me achievements should be medium/hard to get... they shouldn't follow the standard ding 60 and get one. It should involve an amount of time and skill (maybe its argueable that getting to 60 is an achievement, i just think it should be a bit more than that).
I really like collectable achievements, achievements relating to PVP.
The rewards are important! I really think that rewards should not change your stats. As a reward, i feel that ingame vanity items are more approperiate... cool mounts, tabards, dyes, tatoos, titles etc =)
Good luck Frogster/En Masse!! =)