Controls are the basis of the platformers. If your controls aren't downright perfectly sharp, then go work on a turn based rpg. That dealt with, variety is mandatory. The best recent platformers I've played are Megaman 9 and new super mario bros wii. They're completely different, but both offer crazy variety - each megaman level is entirely different from the others, and while that's not true for NSMBW, there are a lot of "unique" levels.
Secret collectibles are cool, but they definitely can't be too punishing - for example, you need to at least have some clues as to where they're supposed to be, like how you know there are 3 coins per mario stage.
Difficulty is relative. I've beaten those megaman games, even doing the "not taking damage challenges". After you got the patterns down, it's doable. However, the player must ALWAYS feel like he's taking damage because he wasn't quick enough or couldn't memorize a pattern. Taking damage or dying due to a completely random occurrence is the kind of thing that makes me want to play anything else.
Other common trap on platformers is not having character development. These games are simple in their core mechanics, so any variety that you offer on controls or playability makes a huge impact. Like when a character learns a double jump skill in a Castlevania game (symphony of the night or the ones for game boy advance/nintendo ds). I don't often play super meat boy or any other of those challenge games because I think timing and precise jumping should be part of a game, not THE game. If you're going for an action/adventure/rpg/whatever platformer, then character development, anything that changes the way you approach challenges, is important. If not, if all your game will offer is platform jumping, disregard this paragraph, and probably most of my opinions altogether, as I will probably disregard your game.
One thing that annoys the crap out of me is HD-lag. Some people can't notice this, but it's very noticeable when I play megaman or super meat boy on a HD-TV. I have some delay between the moment I press the jump button and the moment the character actually jumps. It's milliseconds, but if you can notice it, it ruins your gameplay. Good luck fixing that, as not even capcom was really able to. Maybe a calibration system like Rock Band? Dunno. All I know is that when I want to play megaman, I have to connect my wii to the tv with the old RCA cables, and I can only play super meat boy or similar pc games on my rather old CRT monitor. Again, some people can't notice it, and my tv's lag isn't as bad as some tv's I've seen around, but I can notice it and it sucks bad for platformers.
There are some other elements I could think about, but I guess I couldn't be more specific without knowing what kind of game it is. Maybe if you could explain some more, I could try to be more helpful.
