an_idol_mind
Explorer
A system like AD&D, where you can just ignore a rule you don't want to deal with, is thus "lighter" than a system like 3E, where you have to replace unwanted rules with house-rules.
There are several situations in AD&D where merely ignoring the rules would cause trouble, though. Take out the punching/grappling/overbearing system, for example, and you need another way of figuring out what happens when someone fights unarmed. Take out instant death effects and you're tossing out huge chunks of the Monster Manual and the various spell lists. Decide that you don't want to keep track of what round a fighter gets his extra attack if he makes 3 attacks every 2 rounds and you take away one of that class's few unique abilities. Decide that you don't like level limits and you upset the human-centric balance that is assumed through all early editions of the game. Some of these things may seem like minor points. To certain groups, though, it would be the equivalent of tossing the rulebooks in the trash.
Every system has things that you can toss out with no consequence and things that you need to house rule around if you want to change. The only difference is in what a specific group finds acceptable.