In the past, it was compatibility with new functionality and an element of speed.
But for the 1st time in a long time, I am truly satisfied with my desktop computing experience. Quad core, 2+ Ghz, 8GB of RAM (4.5 of which is rarely used) Raptor Raid 0 array, connectivity, the ability to use any application I throw at it and get it to run in a timely fashion, from encoding videos to running web-apps... It can all be done and done with little to no lag. With the new OSs I get more functionality out of my TV tuner and have turned my system into a multimedia station. Plenty of screen real estate (2x22" LCD's). I am golden. Thousands of gigabytes of information available... My biggest upgrades involve increasing my storage space (and redundancy).
Now all that holds me back is my mobile computing experience. Even with the upgrade to SSD, and it was a great kick in the pants to my tablet, I still don't use it as a proper extension of myself as far as scheduling and communication. It is just too bulky, and the processor, weight, and short battery life hold it back.
iPhone is getting close to what I am looking for, and my (extremely) preliminary experience with the G1 is promising, but the data plans hold the whole thing back, and as with any 1st generation device, I question its longevity...
So in short, when they find a way to make the snappiness of my desktop right now fit into a tablet that weighs less than 2 lbs, with a battery life that exceeds 10 hours for a reasonable price, and when they come out with a phone that has the features I want with a plan I want, my upgrade cycle will effectively be limited to when I get popped capacitors.