I notice that spoon didn't mention pocket pairs in his post. That's probably because their playability is extremely limited. 66 is a great hand on 6xx flop. But how often will the flop be 6xx? And then how often does villain hit something he'll put plenty of money in with? When you mix these factors together, you'll start to realise that 66 is winning a big pot, hmm maybe once every 25 times. I'm probably being generous there too, assuming villain continues vs our set around 40% of the time. It's probably less often unless you're up against a nit who has a very strong range, or a super fish who'll chase gutshots and middle pairs. You're probably gonna be c/f'ing 80+% of the time, so when you do win a big pot, often you'll be lucky if it's enough to cover the last 8 times you played the hand and lost.

78s is a lot more playable than 66, because 78s can flop a lot more straight draws than 66, a straight with 78 will win bigger pots on average than a straight with 66, 78s can flop flush draws, while the flush value of 66 is minimal, and 78 can flop 2pair hands of value.