I've been tutoring Riad the past six months or so and he's been asking me repeatedly why he can't beat the games he's in - he plays $2-4 and $3-6. He's also asked me whether I think I can beat the games he's in and my answer has been always been I'm not sure but probably not. I'm revising that answer to definitely not unless stringently game selecting or exclusively playing heads up and 3-handed.
We had a chance to look at his pre-rake numbers today and he was about a 3 big bet per hundred winner. His actual, post-rake numbers are about a -1 BB/100 loser. So rake at the limits he's playing is around 4 big bets per hundred hands which is obscene. If I'd been paying 4 big bets per hundred in rake at the limits I'm playing, I'd be down millions.
I can only imagine how many thousands of people have tried to become poker players and quit out of frustration convinced they didn't possess the skill. If they started out at lower limits with a limited bankroll, as most probably do, and practiced good bankroll management skills by not playing higher limits unless adequately rolled, they very likely busted. Or just quit out of frustration.
A lot of people who have survived as poker players probably took shots at higher limits when their bankrolls dictated they shouldn't have (myself included). Some of these players likely survived by unintentionally moving from unbeatable to beatable games. I find it interesting that rake, in a way, causes good players with bad bankroll management skills to progress through the limits.
My experience has been that the skill level difference from limit to limit isn't significant, especially in the current poker environment. There is a difference, but it's a small one. We're probably talking fractions of a big bet if it could be measured. But the difference in rake is significant. As much as a big bet or more depending on the limits and site.
I should mention that this all applies to limit hold-em. I'm not sure what the rake impact is at no-limit but I know much higher win rates are attainable.