If you were a WDW oldie and really only spending one day at each DL park and its your first time, which DL attractions we you say were "must do's"?
DL: Indy Jones, PotC, Splash (if it's warm out), BTMRR, Buzz Lightyear Astro Blasters (you can email your ride photo to yourself for free!), Space Mtn, Matterhorn, Nemo (only if the line is short, otherwise I'd skip it), IaSW, Roger Rabbit Car-toon Spin (if you like spinning rides), most things in FL (Alice, Casey Jr, Storybook Canal, Mr. Toad, Snow White, Pinocchio, Peter Pan), and the Castle Walkthrough (if you can do stairs easily). If you have extra time, I'd add in Jungle Cruise, Haunted Mansion, and monorail.
DCA: I feel like there are fewer "must dos" at DCA because many of the rides are similiar to something in WDW (i.e., same basic ride, but different theme/decor). My list of "must dos" would be Carsland (RSR and Mater's), Screamin', Monsters Inc (if you're a Monsters Inc. fan) and TSMM (although this is the same as at DHS). If you have extra time, I'd add in GRR (similar to Kali River Rapids), TSMM, ToT (again, very similar to DHS version, but with some twists), and Goofy's Sky School (think Barnstormer, but with a flight school theme).
In terms of the shows, I would look into seeing Mickey and Magical Map (I thought it was 'meh', but the rest of my group really liked it), Aladdin (but only if you have a fair amount of time because it's long - 45 minutes or so). In terms of the evening entertainment, I really liked
Disneyland Forever (fireworks) and F! (better at DL than DHS to me). WoC is very popular, but you have to be able to see the water surface to see the full show (I don't think it's worth seeing if you can't see the whole show) - I usually go check out Mad T Party instead of WoC. Mad T Party is a lot of fun and it can take as much or as little time as you want.
Same thing for Small World except at holidays.
I generally agree that if you're a fan of MK's HM, then DL's HM is probably similar enough that you can skip it or put it on the second level of priority. I like both versions, but when I ride the DL version, I do miss the Library and the Ghostly Staircase sections. DL's version does have the Hatbox Ghost, so it's worth a look to see that, but overall, I'd still put it on the second level of priority (although, the line is usually short enough that it wouldn't be problematic to ride it while over in the area).
In terms of IaSW, I do think that one is worth a ride at DL. While they do have similar elements, they are actually markedly different attractions on the inside (I'm always surprised of this when I ride the MK version). Personally, I think DL's version is better and worth a ride. If nothing else, the exterior of the attraction at DL is much better than the interior "exterior" of the attraction at MK.