So I did some math on how AD&D-through-3e compares with 4e. A useful assumption for that study is that 4e Levels vs AD&D HD relate as L/2 + 3 = HD (ignoring elite and solo types all through). Turns out there's only a couple of major differences in 4e from there.
- 1: 4e monsters have four times too many hit points, and PCs recover their limited mojo like hit points and spells about four times too fast to compensate for that.
2: All 4e characters are like E6 characters for 3e, only starting at level 6 and fighting the same bloated monsters forever. The math never really changes, they just get slightly more junk to track all the time. It's like every 5000xp your +1 swords stop working and you have to find a new one so you can keep fighting the same monsters. There's certainly no high level spells.
Now, to fix that up, all one has to do is reverse it.
- A: Use quarter monster hit points (round as you will, half bloodied and round up looks fine). This cuts fights down to about 3 or 4 rounds, slightly more as you progress. Monster healing and regeneration effects can be 2 per 5 normal. Monsters get their "Bloodied" status with any wound.
B: Use only 20% normal experience awards for monsters, though full amount (or even double) for the quest bonus. Do not add monsters to compensate. If you have to fill out a 4e-style module, repeat some of the encounters, just with swapped up monsters, whatever makes sense.
C: "Encounter" powers, second wind, etc, recover with a long rest, and that's where you get your free healing surge spend too. "Daily" powers, healing surges, etc, recover with a week of R&R, basically once per adventure.
D: You can't change the math without fixing the damage expressions, nor give high level characters back their AD&D-style awesome without rewriting the whole game, so screw that. E6 isn't that bad of a game anyway, 4e just cuts out all the flavour of it with a bonus treadmill.
- I: You can do a day of adventuring in one session, several fights at ~15-20 minutes each, and leave the party much more time to do some exploring and social bullshit. Classic modules and dungeon-delving adventures become playable because the recovery cycle for PCs is very similar to classic D&D.
II: Because fights are short and individual monsters weak, the issues around great stacks of penalties and conditions building up fade away; fight's already over, monster's already dead.
III: The game becomes more about your strategic limits, what you can finish today. That adds a player-controlled difficulty dial back into the game. Awesomesauce.
IV: Monsters mostly have enough variety to do something different every round, though they have to focus fire to achieve much with it in their short little lives. Characters who are losing too many healing surges need to sit back for a bit.
V: The crappy default Dwarf Fighter can damn near one-shot Irontooth with his "Daily" power. A striker with advantage can do it in two. It's vital to take every advantage you can in such short and brutal battles, making things like marking and combat advantage shine in relation to their complexity.
VI: After four or five fights and a night's rest, you're back where 4e puts you after one fight and a short rest, in the same time. Actual progress made though.