I'm going to hop on with a few gripes that have already been mentioned. First being backtracking. By no means would I say limit it completely, but there is fine line to be walked here. I'd never expect the game to be completely linear and of course its always nice to revisit old worlds with a bag full of new tricks. But if overdone it can become very tedious, especially if making your 4th and 5th trip back.
Cookie cutter themed worlds were mentioned. I'd hate to see a game with out these elemental themes.. A snow filled wonderland is always so inviting, the difficulties antiquated with a fire themed world are always a welcomed challenge, the sunny beach themed water area taking you from your couch to tropical paradise.. But to completely follow the checklist one after another (basic intro world, desert, water, haunted, cave, winter, fire, etc..)would be a bit lack luster. An awesome example in recent gaming has to be DCKTP. Sure there are classic themes through out, but than you're hit with Owl Switzerland, African plains, and Juice Factory! Great variety while keeping some classic.
Now onto swimming. I'd hate to see it removed from the game completely, that wouldn't make sense. But the fluidity of the controls can really make or break the entertainment level here. I'd definitely say its something to be aware of. So many games follow suit on the same old and fall short, leaving us with a section of game we're looking to get through rather than enjoy. Not to keep bringing up DCKTP, but they really did find a way to spice up the water levels in 2d platforming. A fast pace twist while keeping it very fluid. It turned my once most hated levels in the genre into some of my favorite in the game. It can be done!
And the last thing I'd like to add. Coming from an avid Nintendo fan since I was first introduced to gaming. Over the past decade they've really let the level of difficulty within their games slip to a ridiculously low standard. Don't get me wrong, I love them and they're a blast to play. But more often than not I find myself zipping through the first 3/4's of the game with little to no difficulty. It's usually far to late in the game when they finally decide to start testing our skills. And once you find yourself really honing in on them, well that's it, GG and onto the next. There's a lot to be said about how difficult a game is and how memorable it makes it down the line. I'm not sure why Nintendo has went this route, considering the most difficult moments were the ones that really shaped my gaming foundation in the early years. So please treat us with respect and make us work to complete this game! We might curse you along the way, but we'll thank you for it in the end I promise