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There is a generally accepted middleground such words conjure.
When, well, you know.
Not really. The concepts of balance, ease, and challenge are entirely subjective. The only semblance of "middle ground" is attained via aggregate. Which is why Tristam Izumi actually has it right. Asking a mod author to not make something "easy" or "not gift wrap" things, to "offer a challenge" are things that are obvious to you. But not intrinsic, objective, or universal. In other words, they are subjective.
Ask instead for SPECIFIC things, such as "please do not include another ore-doubling machine", or "could you expand upon your tier system to include vanilla ores? That way finding diamonds doesn't bypass nearly your entire tech tree."
It's easier on the mod author because you've given them a specific goal, it provides a more well-defined target for discussion and improvement, and it helps to prevent thread derailment as people argue about definitions. (I'm only poking at myself on that last point, I do that more often than I probably should)
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Back on topic, I definitely like the idea of being a "core" mod with all the metals. That way you can disable the rainbow cave and horrifically obnoxious ore-processing-clogging 10 different types of copper. That might be something you want to bring up with the Forge folks, so you can leverage the Ore Dictionary or something to let you do that. Maybe using IMC to allow mods to tell you how much ore they usually spawn so you can weight your ores appropriately (Like factorization using stacks of lead at a time, or the way IndustrialCraft used to simply BURN through iron and coal).
I'm actually all for having YET ANOTHER ore processing system, because I've discovered a new philosophy about my mods in the wake of the first mod breakage/lapse I've ever experienced—having lapsed in playing modded (out of annoyance/disgust from difficult installs and compatibility nightmares) for a long time before rediscovering mods with FTB.
Duplication of core concepts is a good thing if you can enable/disable them. It gives me more options in-game, which is a good thing. Also, when ModA goes dark or ModB starts crashing worlds, the duplication of core functionality allows me to continue to do what I enjoy doing instead of suddenly refactoring my ENTIRE playstyle because suddenly my house doesn't teleport around hyperspace, or my self-repairing wall stops repairing letting the orges have free reign of my factory. (Again, random examples. I don't want you to duplicate everything, but core concepts like ore multiplication, alloying, etc? Please do!)