I think going from any top-tier to another legacy no-tier would be painful. It's not really an AA thing...

I flew a couple of ATL trips about a year ago. Same thing: short business trips, making an oddball UA routing impractical. I flew DL with no status. I actually was happy to be able to select RJ flights. If there's one benefit to an RJ, it's that it's a great equalizer for a non-elite traveler. ^

I'm used to both the AA and UA websites. Navigating around DL's site was a bit awkward but not awful. I think UA's site would be the toughest of the three for a total n00b to become comfortable with.