1) Not being enough into it. That pretty much goes for all sexual acts and even non-sexual acts in life. If you're not into something, everybody knows and the results are weak.
2) Everything else falls under the "adkjust your approach based on my verbal and non-verbal feedback" category. Some chicks lick the thing like it's a popsicle - I hate that. Some don't come anywhere near deep throating. Some chicks will spend 10 mins on by yambag - the entire time I am silent. But then they deep throat me or jerk it a bit and I'm making haooy noises. Pay attention, don't have a cookie cutter approach and FOR THE LOVE OF GOD, SPIT ON MY DICK
Oh, and many lose stamina real quick. Guess what, it ain't easy pumping away at you ladies either. Work on that