Yahtzee: "I am now making a simple but mainstream-nerdishly hilarious visual analogy of either a valid criticism of the game or just a part with which only I had difficulty, which seems to be about as frequent as Lady Gaga dying her haaaaair. See, there's one now."
[spoiler]Yahtzee: "You also just read that line in my voice as well as this one, and at the same speed as my reviewwwwws."[/spoiler]
Seriously, though, he's right about Sonic in 3D. We already have a little camera/control hassle with 3D Marios, and if not we as in RPM, then certainly we as in the general public just by looking at the sales numbers of 2D Mario games compared to 3D. Now, take the most control-related frustrating moment in a 3D Mario game you've ever had, now set it at about twice the speed and couple it with the fact that you can never move in the straight line that you need to.
It simply does not work. Though neither means/ends is unachievable by either dimensional setting, 2D is simply more inclined towards platforming and performance whereas 3D is more suited for exploration and freedom of movement. The times that Sonic has worked the best in 3D were when you are relieved of directional control except in one direction and the game puts Sonic on-rails (see: Emerald Coast SA1, select sections of Sonic Heroes and Black Knight). Worse, Unleashed bloody well destroyed your range of vision when you used the Boost by blurring everything (visual clutter was also a valid complaint when he reviewed Darksiders, and will more than likely rear its ugly head again when he reviews Bayonetta).
Sonic is 2D only, Sonic Team. No harm, just lesson learned. Get a new IP for your 3D action, rethink stage structure and pacing, and move on.