According to Sega, it's not that the 3D Sonic games are bad, it's just that your expectations aren't right for them.
Sega is facing an uphill battle these days with fans' perceptions about Sonic the Hedgehog. While the publisher's mascot is still popular enough to move a few million units at a time, the games tend to not be well-received by many people. As a result, the character has become the punchline to a bad joke, one representing a franchise that has suffered a steady decline of quality over the past decade or so. Speaking to Eurogamer, Sonic Team's Takashi Iizuka stated that the real reason Sonic games haven't been well-received is because fans of the original games expect something different than the newer, 3D titles can deliver.
According to Iizuka, "Sonic is all about the speed and the platforming. It just comes down to how the gameplay differs when you're playing in 3D and 2D side-scrolling. I understand there are differences in the ways users interpret the 3D side than the fans who particularly like the 2D classic side of the game.
"Those fans are not positive towards the recent 3D Sonic games because we've been releasing only 3D Sonic games in the past couple of years. It's hard to please both 2D and 3D fans at the same time, in the same title for instance."
Iizuka went on to explain that these differing expectations are the reason that Sega has two totally different games in the works. Sonic Colors, which is due out for the Wii and DS, seeks to emulate the daytime sections of Sonic Unleashed. Meanwhile, Sonic 4 will be a downloadable title for consoles that returns Sonic to his 2D Genesis roots.
Personally, I think Iizuka is refusing to acknowledge the obvious: Many people have hated most of the 3D Sonic games because they haven't been good. It's been a long time since Sega released a great 3D title starring everyone's favorite hedgehog. The latest such games have received mixed reviews at best. Sonic Unleashed received many blistering reviews because the werehog sections of the game were so unimpressive compared to the standard racing/platforming levels that took place in a daytime environment. Meanwhile, last year's Sonic and the Black Knight also earned a scathing reception from many reviewers who took issue with the game's short length and "broken" combat system.
Both of the upcoming Sonic games sound promising, but it's hard to be enthusiastic about them because of Sega's track record for the past several years. However, if these games are well-made and fun, then it probably goes without saying that players will embrace them, regardless of 2D or 3D expectations.