1) Waiting in line just to get a fastpass
I know there are lines for RSR and WOC FP, but I haven't really encountered much in the way of a line for other FPs. Were there any other rides where you had to wait in a line to get the FP?
2) Real utensils and plates even at quick service
Not all DLR QS locations have real plates and utensils, but many do. I can remember that Flo's, French Market, Plaza Inn all had real utensils. I think Rocket Redd's and Hungry Bear did not.
Loved the subtle differences in Haunted Mansion but liked them equal.
If you go from mid-September until just after New Year, the differences won't be very subtle.

During that period, HM has the Nightmare Before Christmas overlay, so it's Haunted Mansion Holiday. Even during the rest of the year, there are some pretty big differences between the two rides:
-DLR doesn't have the interactive queue (boo hoo)
-DLR has a real elevator in the stretching room
-The first part of the ride is different (from exiting the stretching room until you reach the endless corridor). DLR's is shorter...and no library.
To the OP, I think you can easily spent two plus days at DLR, even if you're familiar with WDW. There are enough different rides/shows to keep you busy. As others have said, with the addition of Carsland to DCA, it makes it worth it to go to DCA. RSR is one of the best rides at Disney; it may have vehicles that look like the ones from Test Track, but the ride itself is much better (I really like RSR, Test Track is take-it-or-leave-it for me).
A couple of other thoughts that came to mind:
1) DLR doesn't use FP+ or Magic Bands. It's still "old school" paper FP that you pull that day. Thank goodness! They do enforce FP return times (just like WDW). As long as you've entered one park, you can get FP at either park.
2) DLR dining reservations can be made 60-days in advance. All ADRs require a credit card guarantee. ADRs at DLR aren't as critical as they are at WDW, but I still recommend making them if you have places you know you want to eat. If you want to eat at the Blue Bayou, then you need an ADR.
3) Park Hours are typically posted 45-days in advance and often times the EMH/MM and other details aren't added until about 4-weeks out. In other words, the calendar posted on the 45-day mark is often incomplete.