Corrected the list of the 10 reasons to get a 285. From my passing knowledge of AMD's line-up, I am fairly sure all these are available on the 280.
10. DX12 + Mantle
9. Powertune
8. GCN
7. Cool 3rd party cards.
6. Trueaudio
5. Eyefinity
4. Free sync
3. Performance
2. Price $249. So probably £239.
1. Free games. Alien isolation and Star Citizen added to Never Settle.
Only the 260X and 290/X support true audio, so the 285 adds that. Also the 285 doesn't need the bridge for xfire (like the 290/X). But those are the only things it's actually adding to the 280/X, and then it looks like it perform the same or worse and consumes the same power (or a tiny bit less).
So it adds nothing at all that doesn't exist on at least one other card, and doesn't even bother trying to challenge maxwell (the 750/Ti) in efficiency.
I was hoping for more.