Everyone will tell you that practice is important. They are right, of course, but just as important is the right practice. Once learned bad habits can be very difficult to break. Especially when guitarists rely so much on their instinct.
Overdoing the Vibrato
Guitarists often make the mistake of treating vibrato, the vibrating of notes, like a coca cola bottle they want to fizz up; vibrating it with less mind on what it sounds than what it looks like.
In contrast, you'll probably struggle to notice a professional playing it all.
They know that the way to play it is with a relaxed hand that plays in time with the rest of the song. They also only tend to play it only when appropriate. By doing so they help to maintain its value as a special effect.
Forgetting About Time
Time can be a problem even for relatively experienced musicians. They learn the chords and the scales. They add special effects and solos. Rarely do they ever think about playing in time. If they are any good, and have good people around, they will realize after a while and make the necessarily adjustments. But some people aren't that lucky. Having said that there is a solution: buy a metronome and practice with it regularly.
Visualising the Fretboard
Of course the key to playing any instrument well is to slide from chord to chord like it's second nature. The last thing you want to do is to have to think about it. It will ruin your timing and confuse your band mates. So why do people struggle with this so much?
The problem is they rarely take the time to visualise the fret board. Where are the notes? Where are the dots? What are the patterns your finger make for each note? If you want to be good at the guitar, the answers should come to your mind in a split second.
Train Your Ear
If you train your ear to know exactly what note and scale you are playing, you'll be able to know know where you are going wrong. For example, if you're playing a bum note. While even professionals make mistakes, they usually have the advantage of correcting it straight away.
It will be so quick you may not notice it in the first place. If you have an untrained ear, however, you may continue making the mistake and then people will start to notice. One way of training your ear is to always use your ear when learning a scale.
It's very tempting for guitarists to take short cuts. You don't want to learn the scales and how to read music. You want to sound like Jimmy Hendrix or Jimmy Page. You want to play your favourite songs. After all that's the one thing people ask you when they hear your are learning.
And if you look you can find simple ways of playing otherwise complicated songs. However it won't necessarily make you better player. What will is knowing the chords and the scales and having good time. Concentrate on this and you will become a good guitarist.