Friend of mine growing up is an assistant head college at a local mid-major where I use to live. You will occasionally find him at HS games during playoffs to check in a few guys that he has his eyes on (the whole cream rises to the top thing). Paraphrasing his word's, "The hardest thing to figure out has really nothing to do with talent (the perfect swing as you mentioned), and everything to do with the right mental make-up." He looks for grinders with a fun, competitive, and respectful personality.
My ideas / thoughts were more on the physical side of things. Yes, I agree the mental side needs to be extremely sharp. Toughness, competitiveness, grinder, student of the game, ability to observe, etc.
Sometimes I see kids out there that are 5'7 - 5'8 that weight 150 - 160 with not the best of swings, but are scrappy and get the job done. They always find a way to get on base and yes they are dink and dunkers with the occasional double, but they have a lot of fight in them. However, I don't see them getting an opportunity at a high JUCO or even mid-major D1. I guess maybe at a small D1, but I think you get my point.
Pretty much sums up a good JUCO/D1/High D2 need to have the total package. Physical size and strength, good swing, be able to hit off speed and pitches that move, be mentally tough, want to compete and grind. Can not afford to be missing anything.