We flew internally with Delta last year - Dec from L.A to Missoula (Montana) via Salt Lake City and in January from Missoula to Vegas.
I think we were able to pick our seats after we'd booked.
My tips -
If you are travelling with children enquire at the boarding gate with the attendant if you could board early. They announced at L.A that parents with small children could board early but they wouldn't allow us on early with 10 and 12 year olds. That sucked big time as everyone boarding had copious amounts of bags and hogged all of the over head lockers (to avoid paying for baggage I assume). But, when we were at SLC I enquired again prior to the boarding call and I was told there was no problem with the kids in boarding early. Woo hoo for us - I got to put my bag full of spare clothes/wipes/snacks leftover from the stopover etc in the overhead compartment
Also, we paid $25 per bag for checked baggage. Leaving Missoula we were 3kg over on one bag. We were told it would cost an extra $125 on top of the original cost of the bag to check it. So, we pulled things out and rearranged stuff between our 4 bags to avoid the extra cost. Every other bag was under the limit so it didn't really make sense. The man at the counter did not suggest moving the stuff between cases, either.
We did not get snacks on our flight to SLC due to turbulence and the seat belt sign being on for most of the trip. By the time the food cart got to being brought out they got to the first couple of rows then it was time to put the belts back on to prepare for landing. We were two rows behind where they stopped serving
We could not board early for our flight to Vegas so were stuck with scrambling with overhead locker space - good thing as I had decided to pack as little as possible in the kids' backpacks for that very likelihood.
The only gripe I have with Delta at the moment is that I am unable to enrol hubby and kids in their SkyMiles program. I keep getting an unable to process message and so far have had no reply to my emails. I wasn't aware of being able to sign up each person when I booked originally as I thought all tickets would go on the one SM account
Oh, one really important thing - always carry your passport with you at the airport even for internal flights. I thought I'd packed mine in my luggage and thought showing our drivers licences would do - we were almost hauled off to the interview room for 'interrogation' until I managed to find them in a carry on bag! I'm sure many of you frequent travellers know this but it was my first time in the U.S and hubby hasn't been since before 9/11 and it's all changed in a big way.