People have spoken about theory already, and for me it was pattern recognition when learning scales and modes. It becomes easier once you realize that one scale is the same pattern as both the same scale in another key, or part of a mode.
This is not so much a revelation, but something I learned the hard way. When I was 16 or 17, I got into Dream Theater something bad. And all I did was alternate pick fast. No method to the quite literal madness. It wasn't until a good friend of mine whom is a superb drummer said to play it exceedingly slow on the metronome and work on bringing things up to speed did I really start to pay attention to what exactly I need to do.
Last thing, I took jazz lessons for a few years with a superb guitarist. Biggest lesson? Listen more than you speak. I suppose that goes for a lot of things in life, but learning how to listen to him, both his voice and playing opened my eyes.