Subjectively nicer styling, landscape or portrait orientation, funkier graphics and revised animated lane assist. Undoubtedly larger internal memory for future mapping updates for the time being (Garmin mapping bloats year on on year) without having to resort to an SD card.
There are 'added value' features such as winding road routing (would rather decide myself thanks), tyre pressure monitoring (can I get by without it...) and service recording.
Like all modern consumer products; they're only top of the range for an instant and in this throw away, must have it, World will probably sell well. I'll struggle on with my 660 for a while to come, does the job well and to be honest the StreetPilot 2820 I had a while back would still do everything I really need in a sat nav, the rest is just 'window dressing', some of it useful, some of it I question why they bother other than to add perceived 'value'.
If you must have the latest gadget or you recognise one of the new features as something that you would really make use of, and you have the cash to spare, then fill your boots. Otherwise the 660 does a good job albeit having, if you want all/most of Europe to hand, to use an SD card for the mapping these days.