I have become much more sparing with my house rules in recent years, not because I don't want to fix the system or because I'm scared of breaking game balance, but because I have come to appreciate the cost of house-ruling. Every house rule requires players to learn it and remember it in play. That's brain-power that isn't being used for role-playing, tactics, or simply appreciating how awesome my adventure is. 
Therefore, I now make a point of weighing the benefit of each house rule (in terms of improved gameplay, atmosphere, verisimilitude, or whatever) against the cost.
I have little hesitation in ruling that such-and-such doesn't exist in my world, since such exclusions don't come up much in play; the players just have to remember not to write "warforged artificer" on their sheets during chargen. On the other hand, I am very reluctant to alter (for instance) the combat rules. Since combat comes up more or less every session and requires a lot of brain-power to begin with, I don't want to make the players remember any more tweaks than absolutely necessary. If I do change the combat rules, it's usually with the aim of simplifying or clarifying the system.
More and more, my solution to mechanical issues in the rules is to take advantage of 4E's exception-based design to make the problem moot. For instance, I hate the magic item daily power system; I think it's clunky and way too complicated. My first impulse was to work out a house rule that simplifies the way those powers work. For my next campaign, though, I'm going to address the problem simply by throwing out the existing magic item list, using inherent bonuses, and handing out only homebrewed items which don't have daily powers.

Therefore, I now make a point of weighing the benefit of each house rule (in terms of improved gameplay, atmosphere, verisimilitude, or whatever) against the cost.
I have little hesitation in ruling that such-and-such doesn't exist in my world, since such exclusions don't come up much in play; the players just have to remember not to write "warforged artificer" on their sheets during chargen. On the other hand, I am very reluctant to alter (for instance) the combat rules. Since combat comes up more or less every session and requires a lot of brain-power to begin with, I don't want to make the players remember any more tweaks than absolutely necessary. If I do change the combat rules, it's usually with the aim of simplifying or clarifying the system.
More and more, my solution to mechanical issues in the rules is to take advantage of 4E's exception-based design to make the problem moot. For instance, I hate the magic item daily power system; I think it's clunky and way too complicated. My first impulse was to work out a house rule that simplifies the way those powers work. For my next campaign, though, I'm going to address the problem simply by throwing out the existing magic item list, using inherent bonuses, and handing out only homebrewed items which don't have daily powers.
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