I would say the feel of the rooms is the #1 difference.
I've stayed at GF, CR, WL, Polynesian, AKL, BC and BWI. Multiple times at all of those except just once at WL (we LOVED it though!)
This summer we spent one night at POR b/c we drove all the way thru and arrived earlier than expected. We had a res. starting the next day at AKL but when I called it was "sold out" for the night I needed to add... All I could get next door at AKV-Kidani was something for about $389 and that was out, pricewise... anyway, I chose POR from what was offered over the phone. The grounds ARE pretty and the pool was fun for my dd and dniece (so in those 2 departments I would basically say NO difference) but the room was very, very basic. The furnishings, fabrics, materials and layout was just not what I'm used to or what I want. I won't ever book a moderate at Disney again. I would stay offsite at a new Holiday Inn or Hampton Inn before I would do that, I think. Or just stay home.
WL and AKL are the lower-priced Deluxes and to me they are head and shoulders above what we experienced at POR. Incredible theming, great pools, tons of fun...
The food court at POR, to me, felt like something in one of the theme parks. Lots of food choices, yes, but VERY crowded and bustling and NOT in a good way.
If you're debating b/w Mods and Deluxes, I would recommend securing the best deal you can get at AKL or WL vs. going w/ a Mod. There is a lot of magic there
Also want to say that I do one day want to stay at POP Century. Looks sooo cute, modern and clean. And not trying to be something it's not