I think the "big" changes are under the hood on this one - things like the built-in bricking if lost/stolen and airdrop. I'm hoping the security stuff will finally push my employer to allowing iPhones for corporate email. Still trying to get used to the colour scheme by Fisher Price, and the fact that some buttons have been moved for no apparent reason (which is one if my biggest frustrations with anything Microsoft).
I'm going to wait for the chaos to die down before getting a 5s (which is due on my "every other" upgrade timetable). I'm looking forward to the fingerprint unlocking, but otherwise it's just "time" for me to get a new phone. The two-year old battery on my 4s is burning pretty fast these days.