I'm not sure I'd say there is ONE big difference, but there are several smaller ones.
1. The bed size is the same - full, except the family suites at ASMu.
2. Teh room size at mods is larger. I'll get backk toyou one that one.
3. The All-Stars (AS) have a 13% tax, the rest of WDW is at 12.5% (Used to be 11%) The AS are in adifferent county.
4. The mods hav a dorm fridge, hair dryer, coffee maker. AS do not. Pop has a hairdryer. At any value you can get a fridge for $10 per day.
5. The pool/ hot tub. as mentioned above. The bigger resorts in any category have quiet pools, all have kiddie pools. Two of the values have water play areas for the tikes. Theming of mod pools is more elaborate.
6. At the values, you are expceted to bring a towel from you room to the pool, at mods and delux - there are pool towels to use AT the pool. (this is a double edged benefit, IMO, as folks like to 'stake out a seat' by leaving towels on them, even after they leave pool area at teh higher end resorts. That can make it hard to get a seat).
7. The theming at mods is more elaborate, but only values have Disney characters in the theming. Look at each resort for your preference. All Disney resorts are quite clean, freshly painted, well kept up.Though the more you pay, the more you'll get things like granite counters, more counter space, double sinks, better furniture, more hidden Mickey's in the room, etc.
8. Transportation. All of the mods - except POFQ are VERY big. They also have a LAKE in the middle of them, so getting around the resort can take a LONG time - esp. if you are a slow walker. You can easily lose 20 minutes getting from your room to the dining area/main bus stop/pool. Also, the buses to the parks all loop around the resorts, so it takes a lot longer to get tofrom the parks if you have to make five stops along the way. Even POFQ shares buses with POR. The 3 All-stars share a bus - but each resort only has one stop. Pop only has one stop. Pop is also a big resort -so your room could be a bit of a walk,but still usually better than at most of the mods. (no lake) Some folks love walking across the lake's bridge each day, and along WINDING pathways. No resort pathway is ever DIRECT. To sum up - transportation at the values is actually FASTER! in most cases.
9. At mods, the upper category of room is the 'view' - water view vs. standard (=parking lot). At values, the upper room category is the location - near the main building.
10. Both resort categories (value and mod) have exterior exiting rooms. Deluxe rooms have an interior corrider.
11. The resorts with 'lakes' have water activities: fishing, boats you can rent, sometimes paddle boats, sometimes horse carriages, bidges. Pop has a lake, but it's not in the middle of the resort. POR and POFQ have a free shuttle boat to DTD, otherwise all mod and value transportation is via bus.
12. Most of the mods have table service restaurants (not POFQ). All values and mods have food courts. The bigger resorts have better food courts. Pop has one ofthe best. CSR alse has a very good one, but mostly they are similar.
13. When leaving a park - the value and bigger mods have the farthest bus stops - generally. WDW plays withthe location of the bus stops, and sometimes the mods are closest, the values are nearly always the farthest, but the difference is not huge in terms of physical distance. the distance to the 'value' bus stops is more psychological. You may feel more peasant-like.
Or you may feel the 'deluxe' resorts are more 'snobby.' I've heard both sentiments. Often the same folks stay at all three categories.
14. For some reason, folks on the value buses tend to be much more friendly, at least in my experience. This can be a plus or a negative. Sometimes you get great help and kindness, other times, I just don't want to sit next to a 'talker.'
In case I missed anything, WDW's website has a pretty good chart.