Here are some common problems and solutions when playing guitar, this is aimed mostly lead, in particular shred guitar, so if you are not into playing boring repetitive fast sequences followed by interesting phrases, please walk away! The first part covers the problems of the left and right hand, the second part covers problems of both hands.
Problem: Rolling wrists. This will create an illusion of playing fast in the beginning but at a certain speed say above 190bpm will require finger independence that the wrists won't be able to pick up.
Note* this is strictly sequencing we are talking here, it is okay to vibrato/bend to use wrists in fact that gives one more control.
Solution: practice finger independence, try to keep the wrist as straight whenever can, more precise finger movements as opposed to wrist movement.
Problem: using fingers instead of the wrist. Sure, the fingers offer better control than the wrist, but they have to hold the pick!
Solution: pretend the fingers holding the pick is a wrecking ball and your wrist is the string of the pendulm, and the forearm is the top part.
*Note using the elbow as a pendulum pick is Okay, as long as it can be done and one does not feel pain/are not tired and can do it at a constant motion without straining, if not, consider using wrist.
Problem: anchoring the pinky finger/other fingers on the knob/parts of the guitar. This creates so-called reference points that allow one to find strings quickly at beginning stages. However, this really limit's one's speed or one has to work so much harder to achieve the speed playing unanchored. (Debates all around)
Solution: try to curl the unused fingers on the right hand; this will take away any tendencies to anchor the pinky or other fingers.
If one is really dedicated, float across all six strings, there are various degrees to this ex. Some leave their forearm on the guitar, what is essential however is that one should always float the right hand, when not palm muting. Because one is not hitting the other strings, there is no point palm muting those strings is there? (when playing accurately of course, and live performances aside, invest in a noise gate or something)
Problem: pain!, (one is probably doing one of the problems above). And no, the answer is not relax, because that is complete and utter B.S. ala Jamey Andreas, I really want to see her play 16 notes 250bpm on the electric guitar, if such video exists, feel free to call me names and point and laugh and tell me what a pathetic douche bag I am.
Solution: one should analyze ones playing until the most efficient technique is found, for example, if one is feeling the tendon pain on the left fingers, it is probably that one is using more force than required to fret a note, try use the least amount of force to let the note ring out perfectly.
Problem: inconsistency. Left hand and right hand is not in sync. Or impulsing the notes and pray it would be the right ones.
Solution: Finger independence. Muscle memory, a.k.a. Not thinking when playing to see where the note lands next. If one subconsciously memorized the notes, by playing them slowly and correctly hundreds of times, then there should not be a problem as to where the fret hands lands. Play at a much lower speed (ex if 190bpm then go to 100bpm) and play evenly and consistently. This is not how one develops speed (not all the way at least), but consistency and evenness, which is much more important, Trust me on this one!
Problem: lack of speed: lack of speed usually means one is doing one of the problems above.
Solution: practice getting the notes even/consistent and without pain in the body, and usually speed comes. Take the playing slow concept mentioned above, and play the notes really fast, (yes impulsively fast), don't worry about the sloppiness, because the slow playing is taking care of that problem. This is similar to testing how deep the water in the pool really is, and allows one to gain an insight on what the notes playing fast should sound like. This technique and slow perfect playing combined is the real way to develop speed/accuracy.
Well that is all for now, please, feel free to comment.