Welcome to Librarium Online!
Ive started orks not long ago and sincei gto a lobba i thought it was time to update my list. Any comments are welcome.
twin-linked rocket launcher
Nob w pk and bp
nob w pk and bptotal =500 pnts
Nothing more funny than 3 inquisitors with sanctify when facing deamons. They cant do anything while you stand in front of them having a tee break.
It's actually a little difficult to say. 500 points isn't much to work with.
I never take a warboss without a powerklaw. If you're not going to give him a powerklaw, you probably shouldn't use him, but I'm not sure what to replace him with. You could field a big mek with a kustom mega blasta for 50 points, just to take up the spot.
Groups of boyz usually work better with special weapons, especially if you plan to shoot with them, but again, I'm not sure how you want to re-work the list.
Again, it's difficult to say what to do, because the list looks fine overall. The only issues are the warboss without a powerklaw, and the lack of special weapons in the units of boyz.
Here's how I would run the list:
Warboss w/powerklaw, cybork, 'eavy armor, bosspole 
20 slugga boyz 
20 shoota boyz w/2 big shootas, nob, powerklaw, bosspole 
Killa Kan w/grotzooka 
Rokkit buggy 
Although it's a bit risky in close combat, I'd actually attach the warboss to the slugga unit, instead of giving them a nob. A nob would work better, of course, but this is the best way to get use out of the HQ and still have enough points for the other units in the army.
As I mentioned, your original list can definitely work, it's just a shame that the warboss isn't going to be as effective as he could be, and the unit of shoota boyz doesn't have special weapons. It probably isn't crippling, I just think they'll work better with them.
In any case, good luck!
"Any job worth doing, is worth doing with a powerklaw."