Any system, any edition, any setting.

I'm curious to see what rules people have encountered over their gaming tenure that have been really awesome and which ones have been met with seething hatred.

Why was the rule put in place? Why did you love it or hate it so much? What became of the rule, primarily if you hated it, did it get removed...or...?

"The room is spartan in its furnishing."

An unofficial house-rule brought in in every system my gaming group have ever played. Every room in existence from a peasants out-house to a lords orgy chamber is now 'spartan' - because we all tend to overthink things and spend hours inspecting absolutely everything the DM mentioned because "Why would he mention it if it wasn't important!"

*proceeds to take 20 on Investigate check on pair of socks*

"If your action is awesome, you get a +1 awesomeness bonus to the roll."

I instituted this house rule in my 4e game as a replacement for the usual expertise house rule. And it's a pretty awesome house rule.

I really like the usage of Crafting-only XP pack rewards in D&D 3.5/PF.

The "spend XP" mechanic to create magic items was always feelbad, and left the possibility that the crafter might fall behind a level, which is not fun. Doling out rewards of "crafting dust" in a treasure chest, to spend instead of XP was always a great rule. It let the crafters craft, and let the DM control the output.

Favorite:
Out of combat healing is maximized. It's only in the stress of battle that some of the healing energy can be wasted, represented by the random roll.

Mindless undead are always neutral (unless it serves the story that they be another alignment).

There is no Conjuration (healing) subschool. You don't pull (healing) from the aether. Healing spells are Necromancy, just like Inflict spells.

You heal (Con modifier (min 1) x level) hit points with a night's rest.

Least Favorite:
Any called shot rules, or critical fumble tables.

Ha! I told you all it wasn't just me! Go team necromancy!

Im in team necro too!

Favorite:

Giving out EXP for good roleplay, not just encounters. I think it really makes players feel their roleplay is more valued, which it really should be.

~SE++

Oh god, I hate that, absolutely revile it, for two reasons.
First of all, I am naturally competitive, and I will easily get irate if I feel someone is unjustly getting benefits over others (especially if that other person is me). It's unnecessary tension.
Secondly, given that this is Dungeons and Derivatives, I think XP should be level across the characters (except 2e and before I guess where it's a feature). Give them circumstance bonuses to their actions or similar things that don't garner permanent advantage.

Some characters get more done than others outside of combat. And I didn't say to hand out full levels for it, just some EXP to show the players you appreciate their efforts outside of number crunching combat. Personally, I think combat is the least interesting part of the game, and that it's called a Role Playing Game for good reason.

~SE++

Either the XP is relevant, or it isn't. Characters are balanced around (in theory) being equally powerful at the same level. Besides, I don't see why it matters how some are more useful outside of combat, since they get the same amount of share from adventuring. Wouldn't that mean they just get overall more experience period?

So, uh. Why play D&D then?

For the narrative.

~SE++

Because that's not a thing found in other roleplaying games?

It is, but D&D is the most widely known and easiest to find others to play with. Also, I enjoy that house rule for nearly all systems.

~SE++

Rewarding players in any fashion for creativity and well played narrative has always been apart of games I run, because to not reward those players who bring that magic to the table is to watch your game wither and die.

IMO

Least favorite house rule: Used to play in one campaign / world setting where the DM made it so if you played an innate caster, it had to be a female. He had a reason for it in the storyline, but it just kinda frustrated me that if I wanted to play something like an Oracle or Sorcerer, I couldn't make a male one unless I rolled a percentile of 99 or 100 during character creation for it.

~SE++

Make a trap.

I never got gender/race/class restrictions to begin with. Probably one of the reason I've never gone back and tried to get into AD&D - just seems innately stupid to me. Especial the above solution of 'I'll allow it if you roll really well for it'. That's just bad form on the part of the DM. If casters have to be female in your setting then make them be female - don't give out 1/100 chances for no reason. Stick to your guns.

Or ya know better yet don't enforce rules like that. If your player wants to be a dude mage and there is a 1/100 chance of it happening then whats the problem? The player becomes the 1 guy in a hundred.

My favorite houserule stemmed from the exchange of helping a new palyer create their character:
"What's masterwork?"
"It's something that is better than normal. It usually has bonuses related to it."
"Can anything be masterwork?"
"Yes."
"What about soap? What happens if I use masterwork soap?"

The DM ruled that after using masterwork soap you get a bonus to charisma-based rolls for half a day.

Fan-freakin'-tastic. I now want masterwork everything. I want masterwork trail rations.

~SE++