Why don't Shen and Maokai...

Currently, there are 7 Melee characters in our game without MR/level: Kassadin, Kayle, Maokai, Poppy, Shen, Singed, Yasuo

I can tell you the general thought process behind why we consciously have omitted MR/level on Kassadin and Yasuo, and I can wager some guesses that I'm nearly sure are right for why they were left out on all the others as well.

Kassadin's passive provides passive defense versus Magic damage. While it's not terribly interesting, it is still more interesting than a base stat, so basically any power budget we could spend on making this guy better vs. Magic Damage is better spent on the passive than on base stats.

Yasuo's passive is mostly meant as an anti-burst mechanic to prevent him from getting gibbed by mages. Again, the passive's purpose was already aimed at solving the character's needs in terms of mitigating the damage mages do, so any power we could've given him on this axis is once again more interesting being spent on his passive than on base stats.

I'm guessing Maokai was not originally given MR/level because his ult is such a potentially strong mitigation tool vs. Mage AoE. He was also made in an older time where a larger % of the tanks didn't have MRes/level (most fighter tanks weren't around and of the original tanks, many of them had no MRes/level at the time).

Kayle almost certainly doesn't have it because she is functionally ranged most of the time.

Poppy probably doesn't have it once again because of the passive's ability to mitigate burst damage.

Singed likely doesn't have it because of the insane amount he gets from his ultimate, so that mages can actually feel effective vs. him when his long duration ultimate isn't active.

Shen likely doesn't have it because of the precedence set by early tanks and also because his W is a strong anti-burst mitigation tool.

Now I know this all sounds kinda silly because you can point out a lot of similar abilities on other characters who do have MR/level and the whole thing starts to just sound arbitrary. And there is some truth in that; the decision isn't totally arbitrary since all of these character have other tools in place to service their defensive needs, but they just as easy could have MRes/level and be balanced around that as not.

When looking at these guys and working on buffing/nerfing them though, it feels like a cheap and easy way to give them a power buff by just tacking MRes/level, but it's not always the right answer. Take the Maokai changes I've been working on this patch. It would be very easy to just inject power into the character by adding MR/level, but at the end of the day what this change says is that the character is currently working in a healthy and satisfying way but just needs some raw power against mages to succeed. And in some cases, that might be spot on, but on Maokai that just didn't seem like the right approach. Theoretically, there's only so much you can buff a character before they become imbalanced, so it's most interesting and exciting when you can spend your buff budget on impactful actions that the player is making rather than increasing their stat line.

So all in all, I will agree with the sentiment here that our MRes base stat system feels kinda arbitrary, but when working on a character by character basis, we frequently forego just adding the MRes/level to them so that we can try to give that same amount of power to them in a more satisfying and/or interesting way. Hope that helps understand it.

To be honest, it's pretty inconsistent.

There are two main reasons I've seen:

1. This champion is a tank. They will buy durability itemization anyway, and they aren't hurting for gold. This applies to champions like Shen pretty well. Of course there are a million who still do, but we generally seem to want "Fighters" to have some innate tankiness.
2. Sometimes we give champions really weak base stats to make their abilities or item buys more powerful, and just balance around ratios, etc. Nautilus falls under this category (stupidly slow, bad base stats, P/Q/W/R are crazy)