After browsing various Minecraft Servers, I've only encountered ONE where they had custom achievements, and I feel that it would be a rather nice addition. Such as 'Acquire Ocean Temple/Mansion Map' from Cartographer, or 'Tower of Pimps' which is placing 4 Gold BLocks on an Obsidian block which is something I've ALWAYS done on every server xP.
I swear if Minecraft gets achievements before my 3 year old TTT achievement suggestion does I will cry.
@Carned Deathrun has achievements <3 On topic: I like this idea, I mean there are default mc achievements but I feel like a little achievement system for custom things would be great.