If the game is 3D or 2D, it doesnt really matter, difficulty is the key to how hard or easy any game is, level design structure plays a big role too, lets take good old Sonic the hedgehog 2, from the Sega Mega Drive, and Sonic's wii game (whatever its called i've forgotten the name lol).
Indeed Sonic 2 is hard, but it starts off easy and builds up the difficulty over time, the main things which make sonic 2 progressively harder, is the holes in the landscape, monsters all bunched up in one tight spot, loops inconviently placed, and of course, running out of coins from one simple touch of a monster.
Though i havent played Sonic wii, i cant really compare it as well as Sonic 2. Maybe someone would like to take over for me? ;)