This has been my first LD. I've managed to complete a game and the whole weekend was a nice experience. Here's what I've thought... Before LD21 + you can't possibly make interesting levels by hand in whatever little time you have left after finishing the engine and art, therefore you need to make a game that doesn't rely on levels + you can't make decent 3d games in 48 hours + you can't balance a game in 48 hours + libraries are for lazy programmers
After LD21 + you can make interesting levels because developping with libraries saves you lots of time or you could make a level generator Astar Hell fortress escape + you can make nice 3d games in 48 hours with the proper tools Stratus Under a watchful eye Prelude of the Chambered
+ libraries are mandatory if you want something more complex than a tic-tac-toe like game Gather 10 keys to escape
+ mining games are pretty damn popular Lost in the shadows Caved in Tea powered escape!
+ 48 hour entries can provide an interesting atmosphere Escape from the labytinth AWOL
+ you can write a story-based game in 48 hours with superbe pixelated graphics A tale about life, death and a loser
+ WordPress hates me