Specs and Size for me.
I've had the HTC One X now for nearly 2 years, and coming close to upgrade time so had been looking around anyway, however, started a new job five months ago and was given an iPhone 5 as my works phone.
Both on the same network, so has been a really good test for me to compare the two OS's back to back. And while the iPhone has loads of really nice features, there are so many things it lacks (and also performs really poorly on, like dealing with anything in a poor signal area), which has made me really prefer Android.
The only one thing that I really like about the iPhone 5 compared to my HTC, is the size... it's made it feel massive, and now I've used both side by side, I really notice how much nicer a smaller phone is to use day to day.
Went to look at all the other 'Mini' phones, and they're all the same spec as my current HTC One X, which considering it's nearly 2 years old, seemed pointless to pay for a 'new' phone, when it's no better than my existing one, only smaller.
Then the Compact was announced, with same high-end specs as all the other flagship phones, and fit the bill perfectly!