Android 4.2.2 -> 4.3: any gotchas?

My T-Mobile Galaxy 4S has, for a few weeks, been intermittently telling me that it has a software update ready but I need to take it over WiFi. (The first one came at Pennsic, I think.) This morning I got the notification while I was actually home and could connect to WiFi, so I said "yeah, sure".

Note to self: if the phone wants to use WiFi that's probably because it's a large download. That idea you had that it could do its work while you were feeding the cats and checking email? Um, no. It wasn't clear what would happen if I left before it was finished, so I let it have its half-hour.

It was only at the end that I found out what it was (though I guessed from the file size): OS update. I got a popup with words to the effect that some settings might change and I should back up my data, and would I like to install now or postpone? Yeah, that has an obvious answer.

Note to T-Mobile/Samsung/Google: you could improve the user experience there by, in that message, providing a link to a description of the differences between the current and new versions. Just sayin'. They didn't, so I found a report of this upgrade on this phone on my own. It sounds safe. I can't help noticing that that article is dated November 2013 and I'm only receiving this now. The article is silent about settings changing.

I'm also not clear (and this is not the upgrade's fault) on what data I might need to back up. Anybody know? I use my phone to take pictures which are automatically backed up already. I don't think I do any other content creation, aside from text messages I've sent I guess. Do I need to worry about application settings, like Gmail configuration, browser bookmarks, and stuff like that?