Love this thread. Lot of great ideas. I really hope to see achievements added to the game. Here's some ideas:

Hoarder - Hold over 300 different pieces of furniture at one time
Excessive Hoarding - Fill a room with the maximum amount of furniture allowed
Do a Barrel Roll! - Do a Barrel Roll!
Rare Loot Enthusiast - Hold at least one of every rare loot item at one time (not including consumables, ie: pets/furniture)
Giant Cow Worshipper - Spend a total of 10 hours in the Giant Cow shrine
Noob Slaughterer - Kill a total of 100 different characters wearing the default "noob" gear
Unwanted Guest - Enter a single house a total of 100 times without being invited
Just Browsing - Enter every hat & furniture shop in a 24 hour period without buying anything
Lag Much? - Get Warped to Lag Island (I know there's no island there anymore, it's just water now, but you know the place I mean)
Off Limits! - Get warped for entering an off-limits area
Frog Lover - Own a frog morph and all 5 different coloured frog mounts from the giant blob
Blob Master - Kill 100 regular blobs, help kill the giant blob 10 times, and kill a golden blob and a flame blob
RPS maybe isn't your thing - Lose an RPS match against the same NPC 5 times in a row
Big Spender - Spend 50,000 gralats within 24 hours
Firefighter - Put out 10 fires using the water-filled vases in the Graal Fire Department
Bad Egg - Get hit with a rotting Easter egg a total of 30 times
Don't lose your head - Get "decapitated" on 5 different guillotines