What promises? When did they ever say, "Sonic 4 will play exactly like the Sega Genesis games with absolutely no changes?" I think you're imagining things.
When Sonic 4 was still known as Project Needlemouse, there was a blog post in the SEGA Blogs saying that this was the return of momentum based gameplay. What did we get instead? Automation and physics that kill all momentum by dropping the dpad.
I don't think Sonic 4 Ep1 gameplay should have played exactly like the classics, but I think the basics (physics) should have been based on what Sonic 3 & Knuckles already had and improved from it, instead of being based on the Rush series.
Also, in a somewhat unrelated note, what's with all this obsession with Sonic 2? Sonic 3 & Knuckles had superior gameplay compared to Sonic 2.
Based on the Fan Remix Demo, so far I think I would give it a score in the 6 to 7/10 range, same as Sonic 4 Ep1, since both of them have big issues. S4E1 is a downgrade compared to the prequels gameplay wise and SFR has those ugly visuals, with too much focus on the bacground, not enough focus on Sonic and badniks.