Depends what you're using your PC for. For gaming and general surfing the web and stuff: no. But for stuff like encoding and editing: yeah, there's a bigger difference.
No. Even in the things where the i7 does beat the i5 it isn't a massive difference. In rendering movies, etc. It will around a 30 percent difference, then no difference in gaming. I am comparing the new i5s and i7s, the 3570K and 3770K.
If it is in the same generation, just Hyperthreading and Cache. Depending on what you are comparing there might be more but it is usually just smaller features in the chip.
Hyper-Threading. Desktop i5s have four cores and four threads. Desktop i7s have four cores and eight threads. The extra threads are useful for stuff like rendering and encoding and is what makes the i7 better than the i5 for that kind of stuff.
Hyperthreading, cache and clock speed. For most people, you wouldn't even notice the difference between an i5 and i7. Both overclock nicely and unless you're doing heavy multi-tasking or multi-threaded apps, hyperthreading offers little benefits.