Agreed. I reached street cred 50 very easily by just doing side quests and gigs. I think I was about half-way through with the story when I reached 50 street cred. Getting a level 50 character is another subject. I'm at 41 and I've played the game for over 100 hours. I don't know how you can get to level 50 without an xp farm exploit or using a mod to cheat. New game+ would have really been nice for this
I'm close to Level 50 and most of it has been gigs and crime scenes. But you really gotta scramble for experience every way you can.
For instance, if you incapicate enemies you get EXP, but if you kill them afterwards you get additional EXP. And make sure you do it in as many different ways as you can. A one skill build limits the leveling you can do on other skills.
Of course hack as much as you can in every mission even when you may not need it and make sure you always jack into access points for breach protocol EXP.
Whenever you finish a gig, make sure you stick around and kill whatever enemies are left. Don't get spotted doesn't mean "don't kill". If you're having trouble, just finish the objective up to the point you leave the area and the fixer calls you to congratulate you on your good work. Then go back in and finish up everything in that location for the extra EXP before the location "locks" once you've completed the gig fully. This also goes for the few "don't kill" gigs btw. Just stick to it until you've completed the optional "don't kill" objective, then go back in and murder everyone.