EDIT: I'll split it up into points since its a bit of a pain to read.

1. Graphics
First off the animation is just weird... Sonic moves so strangely, he moves much faster than he animates (which looks weird whilst moving through the air especially) and overall it just looks too shiny for my tastes.
2. Levels
Regarding the levels it seems not to be a sequel but more of a "best of the classics" with the first level being an Emerald Hill clone, next Casino Night (now Casino Street...

Between each level there is an annoying selection screen, unless you opt to continue to the next act/zone (which it doesnt always seem to offer).
The chaos emerald special zones are a bit like Sonic 1 however instead of the map spinning itself you control the rotation of the maze and simply guide Sonic through. There are also no bonus levels like Sonic 3.
3. Gameplay
The game just totally feels like it was made for kids or new video game players. To get through a level you just have to hold right and press A when a little target comes up on a spring or enemy. The levels/acts are incredibly short, however tries to use the 3 act system to get around that (each of Sonic 1s acts were longer, and I personally prefer Sonic 2 & 3s longer 2 acts).
The controls are just damn sluggish as well.

4. Soundtrack
I'd also like to mention the soundtrack, since it was such a big part of the previous games... the music in S4 is either a remix or totally mind numbinglt uninspired, unfortunately.

Whilst I love the originals and people have always asked for a more traditional Sonic game I think Sega has taken it too far by simply putting old ideas back together. I understand Sega probably would never get this right in the eyes of the fans, much like EALA, but really... a few new level ideas would be nice, at least, please... since this is supposed to be a new sequel and fresh start for the ever declining game hero.
Well I'm just disappointed... rant rant rant
