According to the recent LittleBigPlanet 2 FAQ on the Playstation Blog, LBP2 will allow you to communicate with the people who play your levels not just by adding text, but also by adding voice acting to narrate your levels or to use it during cutscenes (that you can also make).
The FAQ also answered the whole backwards compatibility questions, so yes, all the costumes (including DLC costumes), stickers, decorations, and community levels will be usable and playable in LittleBigPlnaet 2.
But unfortunately, any DLC that will be available for LBP2 on the Playstation Store will not be usable in the first game due to the fact that they are using a new graphics engine.
You can check out the full information about LBP2 in the FAQ in the link above.