The spark to the fuse.

[Zeta]

I agree. Someone on Gamefaqs actually did a good description of Sonic's genesis gameplay style:

Speed! Flashy things! Run fast!

STOP!

Platforming time. Just like Mario, only with more control over mid-air jumps! Jumping puzzles and obstacles.

Back to speed! Flashy things! Run Fast!

STOP! Mario platforming time!

Repeat.

Instead, all we get are the flashy runny fast stuff. Boring. A game on rails.

[Omni Hunter]

I suppose it could actually. The only way the speed was used in the genesis games was to pass an obstacle like a loop, corkscrew or go up a ramp.

[SegaSonic]

I agree. Someone on Gamefaqs actually did a good description of Sonic's genesis gameplay style:

Speed! Flashy things! Run fast!

STOP!

Platforming time. Just like Mario, only with more control over mid-air jumps! Jumping puzzles and obstacles.

Back to speed! Flashy things! Run Fast!

STOP! Mario platforming time!

Repeat.

Instead, all we get are the flashy runny fast stuff. Boring. A game on rails.

Exactly! Thats the Gameplay SA1 had and Thats why it has been the best 3d sonic game so far.

[Ritz]

Uh, SA1 was pretty much on rails as well.

[SegaSonic]

Still, it was the best 3D game, and I know Mr.Gibbon can agree with me there.

I say the Speed on Sonic games should be like that on Adventure1, It was fast plus you could control the action. You see, SA1 had it all! sure it had shortcomings here and there but it was a pleasent title, down from all the Animal Bots to the references and atmosphere.

[Frieza2000]

Uh, SA1 was pretty much on rails as well.

Only small areas of SA1's levels were scripted. Sky Deck is a good example. You started out unscripted, clinging to the sides of spinning platforms, then worked your way to a scripted straight run. That ends at another platform clinging area that takes you to yet another scripted ride up and down ramps (or around the side on a rocket, if you took that path). Then you're back in control for a brief, running around that big cannon as platforms get blown up around you. The rafters are all straight and the angle is kept fixed, but nothing's scripted.

Then you're in a wide open area with a multitude of hidden item boxes and enemies, trying to keep yourself from being blown off the ship. No scripting at all. Even the angle is free, if I recall.

You get the idea.

[Ritz]

I'm not saying that the game is completely on rails. What I mean is that while the game gives you plenty of leg room, there are only a select few areas where the game gives you a choice of paths; generally, you are given one and only one path, the occasional shortcut notwithstanding.

Granted, it's still the least linear 3D Sonic game thus far, that much I can agree on.

[Omni Hunter]

Bearing in mind that the level is Sky Deck, it's no suprise that there are a couple of scripted bits. The game does the running for you while the player focuses on the platforming.
Besides, with limited space and some wierd camera angles it's hard to run in a straight line anyway. The only instance where this doesn't apply is when the cannon shoots at you and the camera stays fixed by Sonic so you can actually outrun the shots.

[firemario1001]

I suppose it could actually. The only way the speed was used in the genesis games was to pass an obstacle like a loop, corkscrew or go up a ramp.

Loops actually made you go faster, if you rolled through them. A lot of gameplay elements like these are taken for granted and are mere visual tricks in the new games. Even the Advance games use these poorly in the levels, as the designers stupidly put speed boosters right before them. It's why I hate Dimps' efforts: there's no emphasis on rolling anymore, and the rush meter in Sonic Rush only seems to solidify this.

[Omni Hunter]

Looking at it that way, even the Spin Dash manouver took away the emphasis of generating speed, in Sonic 1 you actually have to think about where to generate speed with a run up to get past loops.
Sonic 2, 3 and so on had similar obstacles but the spin dash just meant you could get past it without thinking.

Not that the dash was a bad thing, it just meant games became more obstacle based than physics based only using the physics extensively in the pinball levels like Carnival Night or Spring Yard.
As you said, physics were generally ignored in the 3d and Advance games and because of that the formula translated poorly in the casino stage in Heroes, because they were never really tried out in 3d.
Sonic Adventure got the formula for the pinball shit kind of right though but the set is completely different as it's actually on a pinball table and not a giant mock-up. Sonic still felt too light on the table though in my honest opinion.