I am also a dedicated Necromancer, and I gotta say Skyrim is a horrible example, but that is not the topic here.
It saddens me Yorick is meant to be LoL's necromancer, as that means we won't get another one. He looks more like an ugly summon of a necromancer, so even his style is not worthy.
I disagree about a necromancer having to be fragile though . A summoner needs to have a healthy and steady condition, or he would just be a target, as you can not expect your mostly dumb summons to protect your hide. For that reason necromancer spell schools often have "tank spells" like a bone mail, life steal or strength-reducing debuffs.
It is still true though that a Necromancer operates more like an evil overlord sending his minions forward and stay in the back lines, as he is a lord to his undead, and a lord does just that. (Unless you play Mount&Blade, where they always charge in first). Forcing him to go in with a Shovel +5 including an empowered melee auto attack is just wrong.
The main aspect I dislike about Yorick is the fact that all of his abilities summon undead with a short life span. Undeads may be mostly weak, fragile, whatever, but if there is one advantage they have over everything else in a fantasy setting it is longevity. Undeads "live" until you break them, no exceptions. Riot reversed that and pretty much included the worst undeads ever in LoL. Also his ghouls are uninspiring. They missed the chance to include different kind of undeads.
A nice change, as someone here already pointed out, would be that Yorick kills minions and has the ability to resurrect them. Further instead of breaking down on their own they should die with a few auto attack hits, so they actually feel more like summons and not particle effects of a slow or lifesteal spell. Last but not least I would wish to be able to summon a bigger number of undeads.
Also I don't understand his ult. What is the revenant even supposed to be before the ulted target dies? The ghost/soul of his ally while he is still alive? Doesn't even make sense and is outside of Necromancy.