I started in beta, have played only one character consistently since launch and finally hit 50 on 12/30/06, so maybe I'm not the best example, but...
I leveled up through critical missions and grinding Assassination missions, always on Hard. Having the Download Mission Map ability is awesome; it gets you where you need to go in as little time as possible (and is extremely helpful in those awful mazes of abandoned buildings in Richland). Join with a faction who have high-level players who are willing to take you on missions with them.
I did this until about level 39/40. Then I went through the entire mission archive, and that took me to level 48 and some change. After that, it was grind, grind, grind until 50, because I was hungry for it.
I leveled up through critical missions and grinding Assassination missions, always on Hard. Having the Download Mission Map ability is awesome; it gets you where you need to go in as little time as possible (and is extremely helpful in those awful mazes of abandoned buildings in Richland). Join with a faction who have high-level players who are willing to take you on missions with them.
I did this until about level 39/40. Then I went through the entire mission archive, and that took me to level 48 and some change. After that, it was grind, grind, grind until 50, because I was hungry for it.