What constitutes a great blowjob: (not in any particular order)
1. Ability to not scrape teeth on penis
2. Recognising what is really turning someone on and continuing to do it
3. Not stopping and starting, or changing technique to often (as this resets the persons excitement level back to zero)
4. Enjoying giving head yourself
5. Continuing until orgasm has been reached, no changing for hand relief or suggesting penetration.
6. Personally I think it's rude not to swallow, but at least gracious CIM and then disposal. Not as I've had, Ergh I hate semen! (though it is possible to have good technique with a condom so I never rule it out.
I love giving head, so have based these on what I find works on women, as well as my personal preferences.
I've seen girls who think they are good when the clearly aren't (which I guess is our fault for lying to them!) and also met girls who love giving head as much as I do and that shows so much in their technique and general attitude.
Would you agree?