The short answer: a lot. The long answer: a substantial amount, my dear, it really is quite noticeable.
These new comparison shots, put together by NeoGAF user Prime Blue, show some key scenes and locations early on in the game, with the 1998 Nintendo 64 version on the left and the 2011 Nintendo 3DS version on the right.
Looking at the 3DS version in isolation, it's easy to get a little bummed when you compare it to, say, Twilight Princess. For those not wanting the absolute cutting edge in handheld graphics, and are just after a faithful remastering fo one of the greatest games of all time, these help put things in perspective.
While we've got you, there were a couple of new pieces of Ocarina of Time 3D info released by Nintendo today. Firstly, the game will include a new mode called "Boss Mode", which lets you take on the game's bosses in isolation (or, if you want, all in a row in the one battle). It'll also include video hints for people either new to the game and stuck on a puzzle, or who are stuck, haven't played the game in over ten years and can't be bothered looking on the internet for a FAQ.