A karma score encourages making poor quality meme posts and comments in large quantities to gain more fake internet points. It's easily abused; Reddit is full of karma farming bots.
No downvotes was also mentioned here, but I heavily disagree. Downvotes, in my opinion are mostly a positive thing. Youtube hiding downvotes was a move towards a "good vibes only, no criticism allowed" type of environment.
Lemmy pretty much meets my ideal in this regard, it has downvotes and doesn't have a broken social credit system.