This is just a casual post karthus post.
Things That are fun/interesting He has a unique kind of off- tank/ mage style of play. he can shove a turret with an extremely long decent sized aoe that can really mess with teams. he has ok poke and a slow. a lot of people instantly think about his ult, but really his normal spells are so good. and yes, his ultimate is a lot of fun. depending when. you pop it you can really fuck with teams.
kind of like a taco fart in the face, eh? fun but frustrating?
I love Karthus, his voiceover is excellent and his Q is punchy. It feels so good to hit Lay Waste on single targets and watch their healthbars drop. His spells are also pretty simple, and for a Gold V like me, simple spells = good. He also has no combos to memorize, like one might do with LeBlanc or Riven. It's all about aiming Q, and some might find it boring, but I love it. :)
What elo ?
I've played as Karthus a fair bit. I never play him if I don't have a Pentakill song in my head (and I need to buy that damn skin for him).
But, for why he can seem frusterating, he's a late game mage. It takes time to build up you tears, plus 10 minutes for Rod since he loves that item, and generally needs a lot items before he can really bring the pain he's famous for down.
When fighting against him, I recommend an artillery mage, particularly Xerath. He completely outranges you, pokes you into oblivion, and I've been tilted by the match-up more than once. I usually win late game, but that early game becomes hell.
Always liked that playing Karthus feels like your playing the final boss of a video game. You're constantly spamming shit out that your enemy needs to dodge. I love that final boss feel from a champion where A.) You're an unkillable Juggernaut like 
who needs a whole army to take out, or B.) You shoot out a ton of projectiles that the enemy needs to dodge. (by this logic Illaoi is like the perfect champion for me since she's both of those things and yeah she is kind of fun.)
