IMO, even though it uses the same engine as the 3DS version it does feel a lot different, and I like the fact that when you use the C stick you can charge smash attacks with it now. Also, the 5-8 player smash is awesome, especially when it's teams. 2vs2vs2vs2, 3vs3, 4vs4. Puts a different spin on the game altogether. Probably the only thing that I'm disappointed about this game is what I didn't like about the 3ds version. No wolf, ice climbers, and lucas, however, DLC makes some of them as a possibility. Master Orders and Crazy Orders is fun.
People have complained about the fact that there are a lot of returning stages and not too many new ones. I personally don't have a problem with the amount of stages and the stages themselves since I like them all and since there is stage builder. It seems like the lack of stages is this way partially due to the fact that they worked on two versions of the game and didn't delay them at all (well, technically the 3ds version by a couple months, but that wasn't a biggie). I've only played the game for like 3 hours just to unlock all of the characters. More characters are available from the get go compared to the 3ds version.
Haven't fought Master Fortress yet.