If anyone here has been monitoring the Smash Dojo site, you can see that this game is absolutely packed with content. Rather than list all of it, I'll just post the link right here: http://www.smashbros.com/en_us/index.html
Just to give you an idea of how much content I mean however, I will cite on notable example. Over the last few months, several characters have been announced, bring the total number of fighters to 24, with half of those being totally new to the franchise. Reading the latest post on the Dojo site, I found that these are only the announced characters, meaning that there are even more characters which can be unlocked!
This game is going to be dense for certain, and I am really starting to believe it will become the best game out on the Wii for a long time.