Just to name a few:
1. the screen on the 17" is obviously bigger, but you can get a high-def screen as a $100 option if you opt for the 17".
2. the 17" has 4 speakers, whereas either 15" has two.
3. the 17" has an extra USB port.
4. the 17" has support for 12.5mm hard drives whereas the 15" and 13" macbooks only support 9.5mm hard drives.
5. 17" has a better cooling system because it's bigger.
6. the 17" is heavier and harder to carry around.
7. the 17" has decreased battery life.
there are a lot more small things that you get both positive and negative with the 17".