Really? You're discarting something that would made the level design more interesting for... Dashing? Isn't that one of the reasons that people nowadays complains that Sonic is a "Boost to Win" game? Because they're ignoring plataforming and exploration to put speed instead.
Knuckles abillities were made so he could smash rocks at S3&K discovering new paths. The gliding and the climb were added so he wouldn't be just a Sonic that goes into different routes at certain times.
So here is what you have:
Tails still is the easy mode of the game since he can fly all the way upwards to the upper path. Knuckles is the middle term between Sonic and Tails, you can climb up and follow Sonic's route which is a little more easier but slower or you can sacrifice a few seconds climbing up to Tails' path which is more difficult but faster, I guess that can put Knuckles in the "hard mode" setting since you have to do more choices, be more careful and stuff. Realizing that Knuckles cannot jump as high or run as fast as Sonic, taking Sonic's route can be less productive than taking a few seconds to take Tails route, but in the same way, Knuckles can't fly like Tails, so... Yeah, hard mode.
And finally, Sonic, he can wall jump which helps him climb walls and find new paths, hidden itens, etc... But unlikely Tails or Knuckles, he doesn't have much of air control while doing this do he is more vulnerable to suffering accidents since he have to time his moves and use his reflexes since he's against the wall for just a few seconds and slowly sliding down. And with a good level design you can create situations like this:
Situations that does challenge the player but doesn't break the speed, the action or the flow of the game. Think on how Megaman X inovated over the Megaman' Series introducing the wall jump. How the levels expanded vertically and were more fun and enjoyable to play, how it had well-hidden stuff that required the player to use his new abillity but didn't destroyed the flow of the game because you stopped going foward for a second and start going upwards to get an upgrade, an iten or in case of a Sonic game, a new path.
With the wall jump each character has uniqueness and now can explore not just horizontally but vertically as well. This helps expand the level design, doesn't make Sonic more overpower than the rest of the characters and helps to create ocasions and bosses that could put that in a good use. An example of that working is in Rayman Origins, especially at the Treasure levels where you had to put your skills to good use.
Also I don't see how an dash could help Sonic at all, it could only be used like in Sonic 4, where you're stopped at some place and use the air dash to get to full speed in no time.
This post has been edited by P3DR0: 15 April 2012 - 04:02 PM