8-10 minutes for a non-elite is not bad for most of the legacy carriers these days. I gave up on UA after 20 minutes. Airlines would prefer that you book online.

I am also suprised that the CSA was able to give you a exit row seat. The exit row is considered a premium seat (along with the front of the Y cabin) and reserved for elite Skymiles members.

Seat selection (except for some bulkhead seats) is available at the time of booking on all DL flights. Some seats are only available for DL elite members. Some bulkhead seats are blocked out for the GAs to accomodate handicapped pax.