Tanks, in my mind, theres a few types of tank, Ones that can peel for allied carries (Udyr, Sejuani, Shen to name a few), and ones who can dive enemy carries (Olaf, Shyvana, Renekton) but basically, a good tank will block skillshots from hitting their carries, and/or MAKE the enemy team focus you first, or to give them a reason to focus you first (easiest way to do this IMO is to dive onto their adc (Renekton, Udyr and Olaf are VERY good at this because of their mobility and their cc in their kits)) You get the idea.
Supports, I wouldnt say its a low skillcap role.(Not taking map awareness into account here) Aside from being on extremely limited gold income, there is a degree of knowing when you should and should not go in. Leona for example, Once you engage, theres no easy escape, Zenith blade can get you into VERY deep trouble, she is a very strong 'all-in' tank cc support, but you have to know when it is safe to engage upon the enemy. Thresh, He has quite long cooldowns compared to most supports, and I have seen countless people just go in on landing a hook just because they landed it. thats wrong, you keep the hook on them for as long as possible because as soon as you pull yourself in, the supress from the hook is gone, allowing the enemy to do things again, same process as Leona's Zenith Blade, if you just go in, you can get yourself into deep trouble.
There is also the art of prediction when it comes to hooking, Thresh is very rewarding for this for 3 reasons: 1) cooldown; if you land a hook, the cooldown of Death Sentence is reduced by up to 50% iirc? 2): Low mana cost compared to blitzcrank's hook and 3) it is completely OPTIONAL if you or your carry want to follow up a sucessful hook.
The skill part doesnt have to be about the champion, it has to do with the player's game sense if you understand what I am trying to say.
Mage/assasains: completely agree with pretty much everything you said, I'd even go as far as saying Mid lane is the hardest lane in the game.
Jungling: like supporting, this role is actually more harder than it seems. Again, you are on limited gold, as camps offer nowhere near as much gold as farming a lane would, you ahve to keep on top of timers, keep your (or their) jungle warded (vision is power). And knowing when to gank; this may sound stupid but hear me out. Theres not really any point in ganking a losing lane, we've heard all that before right? not necessarily true, but the reason why you dont gank a losing lane is (in my view) for 2 reasons, 1) stops your top laner overextending so much (if they're not being silly like most are) and 2) if you gank that losing lane, you may also fall victim to that enemy laner and give up a doublekill (or worse, give them your buffs!). If you try and force a gank too hard, you also run the risk of getting your laner killed.
Also because you are on limited gold, you cannot afford to be waiting around for gank opportunities to occur, if your jungle is clear, and ytou cannot see any ganking opportunities, go and ward their jungle or even better, ward AND counterjungle. Warding the enemy jungle is useful because it can allow you to see ganks from the enemy coming sooner, and allows you to counter it a lot faster, maybe even before the gank happens!
ADC: This role is a very close second in terms of laning difficulty for me. Sure, people thiong you just right click and farm at safe distance, but there is such a big difference in auto attack range between the current ADC champions. Vayne vs Caitlyn for example, Caitlyn is known for having the longest attack range of any ADC (actually not true, as of level 14 and onwards, that title belongs to Tristana), and Vayne (and Draven too) have the shortest Range of all ranged champions. Thus caitlyn can EASILY harass a vayne if she tries to farm, and if a Vayne tries to attack the Caitlyn, before vayne even gets in range, caitlyn can land 2 auto attacks on her in that time.
Because of the nature of the ADC role, you have little/to health or resistances, and you must rely SOLELY on your ability to kite and sustain with lifesteal in teamfights, because if one eveny gets to you with any form of hard CC, you're done, and thus your entire team loses its main damage source. Caitlyn, Ezreal and Tristana are fairly easy to kite with in teamfights, as they have very good escapes, and their ranges help them considerably then kiting. However, champions like Vayne, Ashe and Kogmaw dont have such an easy time, Particularly Vayne, Sure she has tumble, but that isn't gonna be enough, Vayne is THE squisiest champion in the game, but if you can use her well, she will DESTROY EVERYTHING. She has the kit to kite, but the short range and the low distance and the bad resistances and health make it extremely hard. Vayne is probably the most powerful champion in LoL. I can't really say how to play her because I'm really awful with her, but I;ve had a few games (think i posted the end result in this thread somewhere) where i've played even OK and completely destroyed my foes with her. back to the other champions, as you may or may not know, Kogmaw and Ashe have no means of escape outside of Flash, all they can hopet to do, is try and slow with Void ooze, or Frost arrows with Trinity force phage passive and try their best to kite away.