Here is my analysis from playing the 4 minutes on wii u and 2 minutes on 3ds.
I picked Megaman on 3DS and Wii U, and I picked Greninja on Wii U, so here are my thoughts of crap.
All of this will most likely get changed/tweaked as through rummaging through Smash threads on other sites, Nintendo is taking a lot of the feedback players had to heart and may change a bunch of things to make players happy with the game(especially the invitational tournament players who wanted to give their insight about what was good and bad about the game, since they probably got to play it the most out of anyone back stage for a few hours before hand)
-3DS-Let's get this out of the way because I didn't have very much time with it anyways. The circle pad is definitely not optimal. It feels awkward and you jump accidentally all the damned time, not to mention the X and Y buttons on the top and the B and A buttons on the bottom make it feel incredibly foreign. This of course will be remedied when you can go in and change the controls to fit your play style. I even think that the circle pad won't be terribly bad with turning off tapping up to jump. But it will surely take you a couple games/minutes/hours/days to get used to it and not input the wrong direction. The visuals of the game are very nice and they don't really hurt your eyes either like I thought that they might(or at least the settings that were on didn't hurt mine but whatever). The 3D wasn't on because that crap always hurts your eyes unless it is being held perfectly at all times and even then it isn't pleasant.
-Megaman-So, we all know how in to Megaman we all are, and how pumped I was that I dissected the move list of Megaman to try and see what was what(I only got crash bomb and metal blades wrong in placement).
-Ground game-*His movement seems like I expected, not especially fast, but he isn't slow either. He seemed to be at the speed of someone like Yoshi or maybe even Luigi.
*His Neutral A moves (jump, standing and standing then moving) are extremely weak and seem kind of pointless, they don't seem to interrupt an opponent long enough for the next 2 shots to go, you can't cancel them in to any of his tilts or specials, which is a bummer I thought it was going to get you a free Down A(slide) or possibly Up A(mega upper).
*Running A(top spin) Seemed to work really well for me, but I'm sure it isn't really all that great of a move.
*His Down A(slide) covers quite a bit of distance and does knock back kind of far, not enough to kill, but good enough so you can start playing keep away.
*His Up A comes out really slow, it almost has the equivalent power of a smash attack, but you leave yourself incredibly open after you use it. Which makes it seem more like an anti air attack to surprise an opponent rather then anything else.
*His Grab seemed like it had very little range, it also seemed like you couldn't really combo after the grab at all. Grabs in general seemed to happen very fast though.
*Forward Smash seems to be fairly weak unless full charged in which it does pretty good for distance and damage, though it is incredibly easy to tell you're going to try and hit someone with it.
*Down Smash seems to be a "GET OFF ME" move, for an opponent who just won't stop pressuring you. If you hit someone with it they will rack up a huge amount of percentage but of course it has a huge window where you can get hit again. Don't really use it unless you're pretty sure it is going to land.
*Up Smash looks to be an anti air smash for someone trying to play in the air and throw out a lot of aerial attacks on you. Same as the other smashes, it leaves you really vulnerable.
*Rolling/dodging comes out very quick which is nice, but it seems to lag a little at the end so you can't just roll/dodge spam and you can get punished for it.
-Air game-This is where I started to see Megaman shine a bit brighter.
*Forward Air(Flame sword) seemed like a very effective spacing tool, though not very strong and not really a kill move. Was very useful in racking up percentage in succession.
*Back Air(Slash man claw) this felt like it had a shorter range then Forward Air, but seemed to do a little bit more percentage since it hit multiple times, but really only if the other person was in the perfect range for it to hit.
*Down Air(Hard Man knuckle) using this makes Megaman stall a bit in air, or at least it felt like that, I tried it a few times to spike someone off the edge but it has a very odd timing on it as it is slow to come out much like the original megaman games.
*Up Air(Air man tornado) I could not for the life of me see how this move was supposed to be used. It felt like Game and Watch's Up Air where he puffs up twice, but worse. It does very little damage and comes out kinda slow compared to other fighters moves.
-Specials-*Neutral B(Metal Sawblade) once you hit the button he will just throw it forward. I tried throwing it behind me a few times but it just wouldn't work. I then threw it in to the ground and used it as an item, but it seems slightly more effective as a Special attack then an Item. Doesn't do too much damage and goes a kind of average speed.
*Side B(Crash Bomb)this sucker landed a bunch of my kills(mostly because people didn't know it hit them) it does decent damage and goes at a slightly slower then average speed. You can only shoot 1 at a time so you can't spam the sucker. If you try to he will do the same animation but nothing will come out. Using this Special seemed best when accompanied by a Metal Blade after.
*Up B(Rush Coil) almost identical to Sonic's Up B in feel and height.
*Down B(Leaf Shield) You can't move when you start this move up, even when you're jumping you just kind of start falling once you use it. Once you use it you can no longer use any of your A buttons as it will fire the Leaf Shield. It also seemed to be the same with his B moves, if you tried to do any sort of attack it just simply wouldn't work and would instead fire off the Leaf Shield. Making the move seem kinda useless to use at all unless there is some cool combo by whacking people with each leaf then grabbing/mega upper or something after the last one hits.
-Greninja-I didn't pay nearly as much attention when I played as Greninja but he was certainly fun. He seemed to play like a different kind of Sheik. I found myself mostly using his Down Air which is the move where if you land it you bounce on someones head and go back up. It can be done multiple times if your opponent isn't smart enough to get outta the way or block or anything. So it is nice to throw out once in a while.
-Mechanics-*Air Dodges seem to have a little bit of lag at the end of them so they are punishable, so don't spam these willy nilly.
*Edge hogging is no longer a thing if you can come from above the edge.
*Edge running(im making this term up) is now a thing, when you run off an edge your character does a sort of roll and brings you downwards, seems that they want you to have a sort of different edge game, I didn't really get to dick around with this too much.
*Olimar's blue pikmin(at least) when thrown at an Item will pick it up and bring it to you
*Fox can now Side B, jump, and Up B. He now has
AMAZING recovery.
*Lil Mac does not 1 hit kill you with his new meter move. It does 30% flat, and will send you very far, but won't kill you from 0%, I'm guessing it'll kill you at around 50%
*It also seemed to be harder to kill someone at least vertically. Not sure if that was intentional or just the stages picked.
And I think that might be it. I had fun with the game, I really want the game to be competitive and have good things go for it, hopefully some things get balanced/changed for the better, and if not that is okay. I will still enjoy the game.