Yeah, I really miss this when using Android, it bugs me big time.First of, this is the longest post I read in a while.
I've had the Google Nexus 7 with the latest and greatest Jelly Bean. I liked it for a few days, but then I started missing simple things like rubberbanding when the bottom of the page is reached and touching the very top of the webpage to hyper-zoom to the very top. I know these can be fixed with a simple OS update (assuming Apple has not patented these features yet.)
But as you said, a simple OS update is hardware specific which furthers fragmentation. So yes, once you go iOS, you'll never go back.
Installing a custom ROM, from XDA or otherwise, is essentially installing Bill and Jim Much Improved Windauz fer Mac over your existing OS. Its ludicrous when you think about it, and even more so when you realize how hard it is to recover from a bricked or nearly bricked phone.
That being said, I miss quite a few things from android. Subsonic, sharing any type of file I want, tasker, changing native apps, etc. But ultimately the iPhone works extremely well for me.
I don't get the appeal of custom ROMs either. I own a Nexus 7, which works well out of the box, I'm glad I don't have to go trawling through XDA in order to find a ROM that makes my device perform acceptably. I'd only ever keep my device on its stock ROM.