Not sure how reputable GameKult is but they say this game will have:
70 missions
Approximatively 10 hours to finish all the missions, and obviously more to obtain all the medals, hidden items, and other bonuses.
No other playable characters. Only Sonic.
They shifted from an extremely active gameplay scheme to an oversimplified one because they found the original too tiring. Also according to him, deep gameplays and big adventures don't suit the Wii philosophy.
Over 30 mini-games, playable up to four players.
Possibility to have online downloadable content.
Link: http://www.gamekult.com/articles/A0000051320/