I am also a DL Diamond thinking about switching over to AA. My home airport is DTW, so I'd be looking at a connection almost anywhere vs many non-stops I have now. That would be the biggest downside.
Getting rid of segment-by-segment awards was just one step too far. That combined with removing everything from ExpertFlyer put me over the edge to start seriously considering a switch.
I'm sure Delta wouldn't miss me, but AMEX might.