How does the bookmark system work? I noticed it makes certain missions stand out on the map, does it do anything else? Make it easier to vote for them or make it easier to start them?
Once a job's bookmarked, you can find it in the pause menu > Online > Jobs > Host > Bookmarked Jobs.
I'm lvl20, I've played it for over 20 hours, and I have a regular crew from AnimeGAF I play with. I don't grind either. But I'm not having a ton of fun with the missions I have access to right now either. I don't really have any favorite missions or races which are available on the menu. There were some decent co-op missions, but you can't bookmark those, which is another issue with the way the game is designed.
The most fun I've had was just free roaming with friends, playing golf, and completing 10 waves of Survival. Other than Survival, the activities generally give jack all in terms of RP and money. Thus far, GTA Online is a completely underwhelming experience, and I was replying to the claim that the missions get much better "after you hit lvl 25". If after 20 hours of playing normally, I haven't hit that bar yet, it's obvious that it would actually require conscious grinding to get there in a quick way. That's not good design.
I'm glad you've enjoyed roaming around and doing activities with friends. There's nothing stopping you from doing that until you're bored. I think if you don't find something fun after 20 hours of it, you should probably find something else to do. But then again, you do keep mentioning the RP and money aspect, which definitely hints towards your motivations. Sure that stuff is there to be taken on and it's all very competitive, but it shouldn't dictate your experience. I just take issue with the claims of "Boo. I have to be so rich/high ranking before I can have ANY FUN."