For the Edge of Empire campaign I'm using 1st edition AD&D rules, but with some tweaks.
First, we're using stone weight encumbrance from Delta's AD&D house rules. This reduces the amount of calculation immensely, and results in numbers that are easy to visualise. A knight wearing 3 stone of armour, carrying 1-½ stone of weaponry, and hauling 2 stone of gear has a total of 6-½ stone of equipment. With average strength, that means she's moving at half speed.
Since Delta's rules and Gygax's tables are a pain to reconcile on-the-fly, I've done up a set of tables for stone weight carrying capacities and their effects on movement rate: Stone Weight - Weight and Speed tables. (More win from Google Docs.)
In addition to that lovely encumbrance system, we're also using two different Experience houserules instead of awarding XP for monsters defeated and treasure recovered. The first is a modified version of Clinton R. Nixon's Sweet20 experience system, which brings the Keys from The Shadow of Yesterday to D&D 3e. (We're modifying them to suit the different philosophy behind XP in 1e.) The idea is that each character has one or more Keys, which are bundles of actions that grant the character XP. For example, the Key of Conscience rewards a character with XP for helping the helpless or less fortunate, while the Key of the Vow rewards the character with XP when they keep a vow (which is chosen by the player) despite inconvenient or dangerous circumstances.
The other XP system we're using (only slightly modified) is Wyrds from Chimera Creative Workshop. (Yay for the Wayback Machine, since that page doesn't exist anymore!) Wyrds are personal quests or goals that give a roll bonus (that increases with level) when doing anything in pursuit of the Wyrd and that, when completed, give a significant amount of XP. Wyrds are player-chosen and can be anything on any scale, so it could be as big as "save the village from the marauding dragon" or as small as "defeat the goblins guarding that door", or even just "cross this river". What keeps it from getting ridiculous is that a character can only receive experience for completing a Wyrd a limited number of times per game session.
What I like about both these XP systems is that they're player-driven. A player can decide that the "Forsaken Temple" on my map looks like it might be full of undead, and declare her Wyrd as "cleanse the Forsaken Temple of undead". I hadn't decided what was in that dungeon beforehand, and now I have a player telling me clearly what kind of adventure they would enjoy having next. (Note too that there are measures built into those systems to change or add to the details of a pull should a player want things to go in a different direction.) What sort of Key a player gives their character also tells me a lot about what kind of play they hope for—a game with a bunch of PCs with the Key of Bloodshed is going to be completely different than a group with the Key of Diplomacy and similar.
Really, that's the best part. They're pure pull mechanics: the player adds something to their character sheet that quietly tells me "do this thing and I will have fun". GMs in traditional games often have a real hard time soliciting any feedback at all, and end up trying to guess how well their campaign direction is being received. A lot of bored players and stressed GMs is a frequent result, though really good GMs learned how to judge and guess what their players want. With two big pulls being laid on me through the Experience system, I will have a much easier time deciding on where to take the game. I won't have to worry about trying to tell the differenced between players who don't like how I'm running the game and players who are just uninterested in the direction it's going. Now, the players are responsible for not only where their PCs go, but also why. I just have to make it happen and make it interesting.