I was able to play the demo again. A lot. No line, so single player.
I used every character. Played every stage in the demo. You shouldn't judge a Mario game based on the demo, for all we know the good part of the game will be the entire second half, like in 3D Land. But I'm going to judge a Mario game based on the demo.
It's really not that fun. The characters don't move as well as Mario in Galaxy, let alone the perfection of Mario 64. The long jump SUCKS, goes about as far as a normal jump. Not even sure why it's in the game.
Further more, the characters feel VERY similar. Far more similar than the four characters in Mario 2. Peach floats for like half a second. I couldn't tell any difference between Mario, Toad and Luigi, honestly. I really hope they fix this for launch. Being able to tackle stages different ways in Mario 2 using the strengths/weaknesses of the different characters was a very good thing. I wasn't able to do anything different on a select stage with any different characters (I'm thinking high ledges that only Lugi can reach, or a far away platform only Peach can float to). It's always one, identical path.
Which brings me to the level design. It's not that good. The art style is bland, stage design itself is mostly flat and wide, likely to accommodate 4 players, the difficulty is extremely low... but clear pipes I guess? Also, they tend to throw walls at you a lot. Because getting the new cat suit and abusing it's mythical powers to CLIMB WALLS is so fun. Wait, it's actually slow and irritating and the furthest thing from fun. The only fun thing about the cat suit is it's dive attack, because I'm pretty sure spamming it is the fastest way to move your character forward.
One thing I did like was the boss battle. You use the cat suit to climb up these columns in order to jump on the boss' head. Already more creative and fun than every 3D Land boss combined, so it's looking like they aren't slacking on the bosses this time around.
So overall, not very excited at all. Though this is just a demo, and the final game could be totally different, they aren't doing a very good job selling me on the possibility that this could be a worthy successor to the Galaxy games. I just don't see it happening.