One big problem I have with repair systems is that they work too well. This breaks the fourth wall instantly for me, since I can see that my weapon's "HP" is at 40/100 or 40%. My sword isn't a living entity, so I feel like it shouldn't have HP. I don't care about the numbers, I wanna see that the rickety iron broadsword I bought on the cheap is being chipped and broken faster than what I'm used to. I wanna see that my plate helm has a big old dent on the right side from an errant greathammer. If that would be too difficult to implement, then I'd have BM do what Bethesda did, and just 86 the idea. I don't need weapon repairing to immerse me in the game, but if it's there it does need to be done right, for me. If I'm gonna think that my armor might fail me in a fight, I want to be thinking that because of something that is represented in game and not by simply reading it and being told that it might "break". Speaking of which, broken leather could still be used to make things, so I feel like good, solid repairing mechanics would be better suited in games that also feature good, solid crafting. Otherwise, it feels too forced and too 'tacked on' to be of any gameplay value whatsoever. It would all be negatory detraction and would have zero positive impact on the gameplay. I would only ever equalize the "negative-ness", not bring it past that.
CubeWorld (a completely unrelated game) has a cool feature where you can collect pieces of ores to place on your weapon, and it is cube/voxel based so it shapes the weapon into some potentially cool designs. That would be a game where weapon deterioration would make sense, causing you to outright lose pieces and voxels of your hand made sword. In SG, on the other hand, I feel time could be better spent making a great narrative, and having amazing character interaction. Not to mention the combat! Repairing your weapons and armor, as immersive-sounding as it may be, does just the opposite, reminding me that I'm the player, I need to do what is best for my stats, and worry about a nagging system that isn't needed.
Maintenance could be cool, but again, I just can't appreciate it in context to how I think SG is going to be. To me, its quality over quantity any day. I'd rather have a more robust dialogue mechanic than an added maintenance mechanic. I can accept that using a whetstone might make my weapon a bit sharper, and thus a bit more damaging, but again, I dunno how I feel about it in context to SG.
I didn't see much posted on upgrading when I skimmed Madoc's posts, but just to put in my two cents:
I feel that upgrading would be cool and all, but again better suited to a game with an already solid focus on crafting, and smithing, and weapon repairing. It fits into games like Skyrim, because of the systems already in place, but I don't feel like it fits well enough into SG to be properly implemented. Again, I'm more of a "if you're gonna do, do it right" kinda guy, at least when it comes to games.
Sorry for rambling, and sorry if my points have already been said, I'm a bit late to the party. This is all in my opinion, and I haven't read enough of this thread to be taking others' points in mind