What I consider to be the most important aspect of playing is rhythm. Most players tend to ignore this all important skill and spend their time developing technique. Don't get me wrong - technique is very important, but worthless if you don't know how to make your guitar groove.
Using a metronome will definatly help you develop your rhythm skills, but nothing can replace playing in a band. If you don't know anybody to play with you can "join" your favorite band by playing along with their music. When you learn a song, try playing it along with the CD. The first thing you will discover is that just reading the tab and going through the motions sounds lame. If you match the rhythm part perfectly you will learn the essence of the word "groove" which I will define as the space between the notes.
When you learn a solo note for note and play it with the CD, make sure to nail the phrasing and the bends. Nothing sounds worse than a player who plays the right notes but lacks the feel and timing of what they are playing. If you listen to Eddie Van Halen and learn one of his songs perfectly off the CD you will realize how great his rhythm is. I'm not only talking about the chord parts but also the solos. he lands on his feet everytime, and that's what makes his playing so cool.
One thing to keep in mind is that when Van Halen was a kid he played along with Cream records all the time and jammed with Alex every day.
Next time you pick up the guitar, try strumming the muted strings with a metronome (if you don't have one, stop reading this, go to the store and buy one) put it at a medium tempo (around 90 bpm) and strum to the second beat and fourth beat while tapping your foot. When this becomes easy play a simple blues shuffle making sure to accent the second and fourth beats. This is the backbeat of most rock and blues.
Now try playing 1/4 notes, 8th notes, triplets and 16th's while tapping your foot to the beat. Feel the division of the notes and the beat. When this is smooth slow the tempo down and do the same thing. Then speed it up. There are a million things you can do to help your rhythm. If you keep it fun, then you will do it more often, and other people will want to play with you. There is nothing more satisfying than feeling the groove with everything that you play.
Mike O'Malley's instrumental power trio is called No Walls and their latest CD is entitled "World Abroad". He has been playing guitar for almost 30 years and graduated from Music Tech in Minneapolis in 1988.
O'Malley currently has 45 guitar students that range in all ability levels and styles.