Completed decks should be an achievement as well. If it was already there, my apologies. Well done, I like the system. Is it possible to look at others' achievements? Is there an achievement top 10 list or something? Staying logged on for a full week would be a cool achievement. Or maybe a full month...that'd be insane and probably not plausible, especially if the servers dropped.
Or how about cool admin achievements like banning 20 people in a single day! That would make the game really interesting! (Disclaimer: For those of you with a slower mental processing time, this entire paragraph is a joke
I like the achievements, very nice. Should improve retention a little bit.