1.5 Seems like its going to add some pretty cool features to the game. I really dig the weather effects, and the achievements as well.
I haven't really looked into the piston mod, but if it adds new things to the game I could see that being a big bonus as well.
I seriously need to join a good MP server, SP is starting to get a little bland. I'm building all sorts of structures, cart tracks and etc...but it gets a bit boring being the only one "doing" anything in the world.