Home screen widgets is a biggie. Custom ROMS is also a biggie. Replacing kernels and overclocking is big. Real multitasking is big.
I'm not entirely educated on what a jailbroken iphone can do, though.
However, I highly, highly doubt that the iPhone 5 will support 4G. Apple has a track record of rejecting existing technology that will add bulk to the phone or have negative effects on battery life and performance, and 4G will do both. It's simply too new, and a standard has not quite been decided on (although it looks like it'll be LTE). Remember that the original iPhone did not have 3G capability, even though it definitely existed back then.
I don't know. Every US carrier is pushing 4g to the max right now. Apple would be committing suicide by not making the next model LTE capable.