I agree with feppe that 'features' like content aware fill and puppet warp are just gimmicks that few people want. There was nothing in the CS5 upgrade for me. Small improvements in existing tools like edge masking may well be welcomed by a few users, but a reason to pay for an upgrade ? maybe not.
The second issue is just what percentage of potential upgraders that use PS can charge for their time ? or maybe more particularly, how much of that chargeable time is taken doing tasks that can be significantly shorter with the new features ?
Time saved also has to be counted against the time taken to upgrade, learn effective use of the features and adapt to any changes in the new version. I know a lot of people are more productive with older versions that they know very well, rather than having to change their working practices every two years.
I've noticed that when I first use with a new version of any Adobe product there is always that slight adjust as things have changed. But without fail, whenever I then go back to the previous version that I am far more familiar with, I find myself missing things in the new version. Adobe, unlike Apple are very careful not to upset long time users with modifications to how things work and usually consult before removing anything, so adjusting to new version is pretty painless on the whole. With the Adobe products I use that is.
And it doesn't even have to be flashy things like content aware that make the change worthwhile. Little UI tweaks to frequently used tools can sometimes make life so much easier than the headline features
Something that also seems to be completely overlooked in this thread and all other threads on this topic, is that when ever you buy the current version of PS or the entire Creative Suite after
the announcement of the new release you get a free upgrade and Adobe would be insane to change this as just about every software company does this. Their update from several version back was however unusually generous.
This means that previously you could upgrade from 4 versions back, not just 3 and in future it will be 2 versions, so if you are the sort to skip alternate versions, then you still will be able to do so. This isn't even a hidden thing as the product manager for PS, John Nack mentioned it at CS5 update.
And compared to the price of upgrading cameras or buying new lenses/gadgets for one's photography, the upgrade price for LR/PS seems a real bargain even if you do it every refresh.
As for the price being dropped in order to reduce piracy as some people have posited. Yes it will have an effect on some potential purchasers, but not that much in my opinion. People steal much much cheaper music and films all the time. OSX Lion was up for pirating even before being released and Lion is stupidly cheap to purchase. Lion cost me less than £30 and that's for all my Macs.