Edit this later with more stats/info as I find time. I'm mostly focused on crafting and other meta skills and have been since the start. Maybe someone else should run this section.
I do have some comments here at “Which Two Combat Skills Should I Use” regarding skills that are good for leveling other skills. Players that are asking themselves the question of “What is the best skill to start with in Gorgon?” should click the above link to see a breakdown of the newbie skills. The short answer is to just choose either Sword/Psych or Unarmed/Mentalism or some combination of these four skills when starting to see how they like the game with them, and then swap out at level 30-40 if the skills aren't interesting to you anymore. It will takes you days to go from Level 1-40 in a skill but a month or more to go from 40-80 in a skill due to the resource costs. That's how difficult the progression gets, so getting a couple skills to forty to test them all out and see how you like them is actually pretty easy.
It's also worth noting that unless you are hardcore player, the “holy trinity” of tank/healer/dps doesn't entirely work the same in Project Gorgon. You can certainly group up like this, and there's tons of group content in the game, but skills like Priest and Bard are two of the best healing abilities in game, but extremely difficult to level together solo, they also both rely heavily on channeled abilities and generally speaking you only really want one channeled ability. Skills like Pig, Priest, Mentalism, Psych, Warden are good skills when paired with damage dealing skills. You may want to level support skills like these separate from each other when playing solo, however, even if you end up pairing them together when doing group content.