In Crazy Taxi's case, it's the lack of licensed music and locations, meaning that destinations like Pizza Hut have been replaced by generic stores, and the Offspring and Bad Religion soundtrack has been swapped out for sorta/kinda soundalikes. Actually, given how many times we've heard the Crazy Taxi soundtrack in our lives (too many!), that might be a plus.
For Sonic Adventure, the limitation of the XBLA version is immediately apparent even to those who haven't played it before: it displays only in 4:3, with vertical bars (featuring a blue patterned background) on the sides of a widescreen display. A Sega rep told Joystiq that the code for Sonic Adventure couldn't be altered to support widescreen as easily as Crazy Taxi's.