The best DMs are a treasure; they can make anything fun.

I think "good" systems (from a DM's perspective anyway) are the ones that make that job take a the minimum amount of effort; If I need a monster, I can make a statblock, but if the system includes a monster manual that has it for me, I can spend that prep time on some other encounter. Likewise, having just enough lore, such that I can grab elements and inspiration, but not so much that I have to exhaustively research it, is a huge benefit.

From a player's perspective, I guess my priorities would be a system that isn't actively frustrating to use, and one that I can have a pretty good understanding of the likelihood of whether any particular choice will succeed or fail. Doesn't have to be perfect knowledge, just so long as I have the chance to make a meaningful decision between my options.