Black Cleaver isn't even core on Riven, it's a good item sure, but usually not worth rushing over hydra and LW.
No mana also doesn't immediately make a champ OP, nor does having a 1 second shield. The reason you hear the skill argument so often is because it is true, not because boards is full of salty Riven mains.
You also don't spam buttons on Riven, you weave autos into your Qs (preferably with AA-canceling if you know how, which is almost required knowledge to be competitive with her). You don't spam out your shield because it's the only damage mitigation you have besides Q3 and your W. If you don't learn how to animation cancel with Riven, your combo takes far too long and gives your oppenent time to hit you back, and Riven is a squishy champ for a toplaner.
You said you wanted champs with clear weaknesses, here's Rivens' weakness: CC and poke damage. This is because CC shuts her down faster than an ulting Katarina, and if you do any damage to her in lane, it sticks, because she has terrible base health regen and has 0 sustain until she gets lifesteal.
She's an easy champ to pick up, play, and understand what you need to do, but actually doing things correctly is difficult. If you don't believe me, pick up Riven and play her top a few games. If you're right, then you'll be able to rake in that freelo, but you're making her out to be easier to play than she actually is.
EDIT: Actually, if you have a lot of trouble laning against Riven, I do recommend playing her a bit, it will teach you her thought process so you can predict her plays as well as teach you her weaknesses.