Zippers.
When I first started riding pull on boots were traditional. It was a coming to age ritual, you graduated out of jodphurs and you got your first pair of tall boots. The lady at the store helped torture you into your first pair of evil black boots. Then you wore them everywhere to break in and your calves hurt, your feet hurt, but you were proud!
Then came the zipper. Someone figure why not just zip them bad boys on!
Slowly over the past 5 or 10 years the pull on boot has started to disappear more and more. You can find them online, but in retail stores they're special order. Most don't want to special order them, because if they don't fit they're stuck with a pair of boots they can't sell.
The only boots that generally still are pull ons are the rubber/pleather boots and those aren't allowed. (I'm not 100% sure on the pleather).

Problem with Side Zips: They're not a normal feature. Most of the zippers will be down the boot's seam. I was shocked to find an Ovation to have this feature. It's usually a custom boot option. But I'm impressed. I've never owned a pair of Ovations, but at this price I may soon.

Why I Hate Zippers
I had three pairs, different brands, and the zipper broke on me. Dirt/Hair gets in the zipper and you really have to take care of it. I love the look of the pull-ons. Though I have to say side-zip would be my second choice if I had to go with a zipper.