AbdulAlhazred
Legend
Weapons vs Armor was optionalLike Ahnehnois said, if you're going against what's written in the book, on an ongoing basis--that's a house rule. When I run 4E, I don't give XP and the players just level up when I tell them. I also disallow resurrection magic. Those are house rules, even though I don't have them written up in a binder. (Homebrew is when I make up a new class or monster or something; adding to the book instead of altering what's there.) If you played 1E and ignored the weapons versus armor table... that was a house rule.
The Big Binder o' House Rules is just taking this to the extreme.
Here's the thing. What is a 'codified rule'? Lets take your example of XP. We already know that in 4e at least the DMG says basically "give out whatever amounts you want in order to have people advance as you desire." So, there is a difference between that and just advancing the PCs at the point in the plot where it makes sense? Not really. You achieved exactly the same result. I ran a whole 4e campaign that way, we'd just get to the end of a session and all look at each other and decide "eh, that's about enough encounters to go up a level" and I'm sure we were probably just about dead on. Now, if you leveled people for some plot reason with no reference to whether they had 1 encounter or 20, OK, now you're into real house rule territory. The other is just "how I run my game".
Anyway, it seems like largely semantics to me. It just seems to me that most of what people call 'house rules' are more "how you actually employ existing rules and guidelines" to me than actual different rules.
For example, consider getting new spells in AD&D. If the DM say always gave you an extra new spell when you hit a new spell level is that a house rule? How about if he fluffs it as your mage guild gave it to you? Is it still a house rule or is it just RP? It is exactly the same thing... I think the vast majority of stuff is kind of like that. It is at best highly unclear.