Well, I've had the game for a day now, and here's what I can tell about the main two characters I've used.
Palutena just seems straight out bad. It's sad, because I'll probably still try to use her as my backup (Just because I like her), but her default specials just do not have the right kind of synergy. She has two defensive moves in her side B and down B, and her side B is especially situational. Her neutral B is what most people expected from her, being a zoning tool, but it's too unreliable to work well. Her best strengths to me seem to be the appalling range on her up smash and the pure KO potential of her u-air. This forces you to try to keep the opponent above you, but she doesn't really have any moves to put them there besides her throws. Her tilts are slow and have little combo potential beyond her u-tilt, leading you to have to try to play defensively and stack up damage with neutral B spam, which doesn't work well. They are also easy to punish if whiffed or on block. That's at least how she looks to me.
Robin seems pretty decent to me, despite what the Japanese tier lists may say about him. He has a good zoning tool in his side B, and his neutral B is just all around his most useful special. Thunder is quick and surprising, with Elthunder being a stronger, slower variant. Arcthunder and Thoron are definitely where this move shines, though. Thoron can be all around unpredictable if used right, and has great knockback at mid-high percents. It comes out blazing fast and covers the entire width of the screen, making it an extremely valuable tool. However, the main thing I want to point out is about Arcthunder. It seems as though a lot of people haven't realized this, but I believe Arcthunder is a much better tool for scoring KOs than Thoron. It appears to have better knockback at higher percents, and testing it myself, it does indeed seem to work better (KOing around 95-100% as opposed to Thoron KOing at around 105-110%). However, it lacks the speed of Thoron, so you'll have to time it right when there's an opening. Unfortunately, Robin's down B is pretty useless. It has short range, doesn't really deal or heal enough % to make it worth the risk, and has almost no knockback. His up B is average, offering good recovery at the cost of predictability. The two shots below him seem like they could do some interesting things though. Robin's A attacks seem solid. You can "smash" them in midair to use the Levin sword, which has some of the most ridiculously powerful air attacks when they hit. Almost all of them can KO. He has good approach tools in his fair and bair, and I believe (don't quote me on this) that his dair can spike with the Levin sword. His jab is solid, offering decent damage and knockback, though it's escapable if you use it for too long. Robin seems like a character that will take effiicient usage and knowledge of his expendable moves, but they can pay off if used well.
Beyond these two, I've just got a few comments, though I don't know for sure how correct these observations are.
Rosalina seems like she can be a monster. Her zoning is amazing, and Luma can serve as a shield if need be. I played her a bit, and she seems complicated to control correctly, but I have no doubt that it would pay off to learn her.
Marth definitely seems weaker in this game, though I don't think he's unusable. He seems slightly stronger than Lucina to me, having tippers and a spike.
Zero Suit Samus seems to have an all around good movepool. All of her specials seem useful, and her up B can KO fairly easily at higher percents (sometimes on accident when recovering). I expect that she'll see some time in the spotlight.
Ness seems to have gotten some big buffs to both speed and power. He's definitely useable, though I still don't expect to see him do too well due to the predictability of his up B and how easy it is to block.
As many people have said, I expect Bowser to be a major threat. His speed and power are crazy, to the point where one might begin to call him overpowered. However, we're still exploring the game, so I won't jump to any conclusions.