I mean, specifically, that in order to change a rule because you have a "good idea", you have to trust your idea is good at least as much as is needed to try it.
That seems to contradict your earlier statement that you trust the game designers more than yourself.
If my thought process was, "Ok, I'm confident that I'm smarter than the game designers because, let's face it, I'm awesome and nothing I do with my D&D game will ever blow up in my face.", then sure, I'd be contradicting myself.
But that's not how I operate. I think like this: "Ok, I'm a graphic artist, not a game designer. Since game design is their job, they are probably much more knowledgeable than I am about game designing. The designers tell me that I should rule it like X, but I wonder if ruling it like Y would be more fun or better? Why didn't the designers rule it like Y? What would happen if I ruled it like Y? I'll go post on the forums and ask opinions about what people think if I ruled it like Y. Afterall, I'm no game designer and I won't pretend to think I know better than a game designer. But (as someone mentioned) the game designer doesn't know my group and can't plan to cater to my needs. Maybe plan Y will work for me, but I'd like to find out first before I implement it."
Ok, so that's the long version of my thought process. I usually cut it down to: "I wonder if my plan Y idea would work? I don't want to cause any problems in the game, so let's figure this out before I just implement it."
I know I said I'm the smartest man alive (so far, everyone seems to agree), but I'm also humble enough to admit that maybe a game designer knows better than I do when it comes to rules. But if I get lucky every once in awhile and use a house-rule that works for us, I'm not thinking, "Hah, you idiots, why didn't you just rule it like I did you fools!" Overall, I am not afraid to say that they are smarter than I am when it comes to game design. They are!
It seems to me that you are describing changes intended to cause positive change, that have been thought out to various degrees.
When I say, "willy-nilly", doesn't that imply that it was not "thought out to various degrees"? If it was thought out to some degree, then I wouldn't exactly say that the rule was implemented "willy-nilly". Unless if maybe the rule was called Willy-Nilly. Maybe it's a feat or something. I don't know.
An example would be my buddy who is going to DM us in 4e Darksun.
He barely knows the 4e rules, but he's already had us using the 2e Darksun ability score racial modifiers instead of the standard +2/+2 that 4e races get. This means, as a Half-Giant, I'm getting a +4 Str/+2 Con/-2 Int/-2 Wis/-2 Cha score rather than just a +2 Str/+2 Con. He gave absolutely no thought as to how this might affect our PCs. His only reason for doing it was, "I prefer the 2e adjustments."
I'm no game designer, but I'd assume that as useful as all ability scores are in 3e/4e to many different parts of a character, and the fact that no PC in 4e takes any penalties, this ruling might be gimping our PCs in a lot of ways. I figure, the game designers don't have penalty ability scores for a reason. I could be wrong, but I at least gave it some thought and mentioned it to him. He told me, missing 1 point here and there is hardly a game changer.
I just asked him about my weapon size damage because he made my Half-Giant large sized. I don't know 4e rules very well, and when I asked him about my weapon damage, he said, "Uhm, 2d8 sounds good." He came up with that pretty willy-nilly. In fact, soon after I saw the ruling in the PHB that said my weapon damage would be 2d6. He said he'd rather just keep it at 2d8.
A lot of people come up with rules willy-nilly. In fact, I just want to say willy-nilly again.