Here's a few:
- Fixed HP per level (3/4/6/8/9 for d4/6/8/10/12)
- A PC with 1 or more hit point cannot be slain by direct damage. The lowest his hit point total can be reduced to is -1. However, a character with 0 or fewer hit points gains no such protection. This rule applies only to PCs - NPCs, cohorts, familiars, animal companions and mounts all take damage normally.
- Fixed starting cash (75% of maximum by class)
- No multiclass penalties, or multiclass restrictions for Paladin/Monk
- If a character multiclasses into a class with higher HD/Skill Points, he gets additional points to make up the difference
- Charisma has nothing to do with appearance (this has absolutely no in-game effect)
- Encumberance is not used (movement is impeded only by armour)
- No need to track rations or inn/tavern costs (if it becomes important, assume the party has supplies for a week)
And a few we either haven't used, but were going to in the next campaign, or that we adopted late in the current campaign, and were going to adopt from level 1 next time:
- Monetary values less than 1 gp are ignored
- No need to track mundane ammunition
- Gnomish ability adjustments are +2 Con/-2 Wis. Halflings have no ability adjustments
We don't use all of these all the time, and have also experimented with certain rules from Unearthed Arcana and Arcana Evolved (weapon groups are a favourite).
Some of these rules exist purely for flavour reasons (the Charisma thing, for instance). Some exist because I don't like randomness in character management. A couple exist to deal with small balance issues (a Wizard/Rogue is more or less powerful depending on which order you take the classes in the core rules, and Small fighters really suffer for that Str penalty). Finally, some exist to reduce book-keeping (encumberance, ammunition, costs under 1gp).
I haven't seen any great problems with any of these rules, although the higher than average fixed hit points and the one about dropping to -1 hit points do significantly reduce character fatality (except at higher levels, when most deaths are by instant death effects).