Obviously I don't know what's been changed under the hood, but I know my experience.
A17 takes up a lot more RAM (it's been hitting 30%+ on my 2Gb Macbook), acts a bit more sluggish, and crashes more often. I tried to copy and paste the article on "Serpent, Bronze" in the Anchor Bible Dictionary, and it crashed. I don't think it's a fully functional feature (never got that far), but just highlighting that much text and right-clicking brought it down.
I've also noticed an odd thing with the SESB 2 BHS Apparatus. When a variant is marked on the text, mousing over the letter marking it doesn't pop anything up. Clicking on it doesn't open the apparatus, but the "Cited By" tool or "Power Lookup" tool, which is very odd.
Ben