A few huge biggies for me are:

1) no more 9 volt batteries or huge honkin AC adapters
2) Being able to program changes into midi & have your computer make the desired patch changes.
3) A send/receive plug so you can use a cabinetulator on all your patches.
4) just in my experience, the solo pedals sound noisy compared to the multieffects pedal. Probably cause if you use 6 of them, you've got all those extra cables to go through.