"Speed returns"? Seriously?
The original 2D Sonic games were essentially your standard platform games. They had two distinguishing features.
The first (and, in my opinion, most important) was it had a real sense of momentum. Speed was a small part of this. More important was that your momentum affected every part of your movement. You have to accelerate to get to top speed, and decelerate to get back to a halt, or to change direction. The terrain affects your momentum somewhat realistically - you accelerate downhill, slow down going uphill, your jump trajectory is affected by your movement, and the terrain can be used to alter your direction.
Need to go up? Run into a wall with a curved bit on it. Need to get over a hole that's too wide to jump? Run headlong into the half-loop off to one side, and run across the ceiling, over the hole, dropping off the ceiling just as you hit the other side. Huge pit with a bad guy at the bottom, and no obvious other way over? Jump from a great height onto the badguy, sending yourself bouncing back up, and over the pit. Need to get on a platform, but it's too high? Go back a bit, down a little hill, hit a springboard, go back up the hill at high speed, jumping just before you reach the top, sending yourself sailing neatly up onto the platform you could never have reached before.
OK, maybe that's just the way I played it. I always considered Sonic's movement to be less about speed, and more about acrobatics. The speed was just something that enabled the acrobatics, and broke up the platforming sections.
The other important aspect is the level design. While the first levels of each game (Green Hill, Emerald Hill, Angel Island, Mushroom Hill) were basically straight left-to-right running, occasionally jumping over / onto something, most of the other levels were much slower paced, and more complex. You have small platforming sections, with obstacles and terrain that you need to get past, connected together by short high-speed sections. You even have multiple pathways through almost every level, often needing to use your momentum to reach the less obvious routes, or forcing you to take a more difficult route (underwater, for example) if you screw up and fall. The Flying Battery in Sonic and Knuckles is a great example - there are enough alternate routes that you'd have to play the level at least three times to find them all.
Newer games have neither of those things. The games basically play like they're on rails. Aside from running into obstacles before you can see them, the speed (which is stupidly fast in Sonic Unleashed) doesn't really seem to accomplish very much. Just press left and right to dodge around the obstacles, mash the jump button occasionally where you need to hit enemies, and let the game funnel you down a fixed path between the different parts of a level. Scripted transitions everywhere. Walk over here to play an animation of Sonic going around a loop. Hold down to watch a whale destroying a bridge. Entire levels that are basically a giant downhill road, walled off at the sides, with an unmissable wall of springboards at the end which catapult you directly onto another giant downhill road...
The general trend is that Sonic Team seem to think they can remove all the platform game elements, while just making the game faster and faster. Which is boring. If I wanted that, I'd play something else. Or go on a roller coaster.
It's not confined to 3D either. Sonic Rush suffers from the same problem, combined with a tendency to put a bottomless pit in the middle of a section where the level design forces you to be headed right at full speed, inevitably causing you to fall down the hole and die unless you know it's coming. Sonic Rivals is just as bad.