Rhythm
I'm going to mention one musical skill above all others and that's great rhythm. Great rhythm is critical. Rhythm is is a huge subject which we don't have space to cover completely here but we'll start with some basics. People think that rhythm is simply playing in time. This is a tiny part of rhythm. Rhythm is part of everything you play and if you can play it with great rhythm, you'll be a great musician. Every melody, accompaniment, vocal line, solo...everything has an inherent rhythm. It's too easy to pass this off without making sure that we're playing the rhythm properly and in time. All too often musicians are all over the board when it comes to playing and soloing. You want to be in time or be out of time on purpose. It's important that you really take notice of where exactly you are putting those notes. If you're not sure, try this:
Set up your metronome at a pretty slow pace. Try 60 BPM for now. Now try playing a major scale with quarter notes right on the beat. Try to stay with it and see how long you can go without rushing the notes. After a time, most musicians will start to rush it especially with something that they can perform easily. Next, try playing one of your favorite solos or songs at the same tempo. Are you playing in time?? Not so easy is it?This simple exercise usually tells us how much we may be missing simply by playing through pieces without thinking too much about exactly where we are placing those notes. The same goes for playing rhythm parts. Take your metronome and try playing straight 8ths. It's important that you practice this on it's own. Its seems like a simple exercise until you see how far you can go off without thinking about it. If you think you're great, try recording your performance with your favorite DAW. Then when you're done, magnify your track so you can align it with the timeline in the sequencer. Now check to see how often you were right on the beat. How often were you early, how often were you late? You'll find that you weren't consistent as you would think. And, (this is critical) can you hear the difference without referencing the sequencer? Practice this, just this on it's own. You'll start to notice and hear the difference in a short time.
Listening
To most this may seem obvious but it's amazing how many musicians fail to listen actively. That means not only to pay particular attention when playing pieces but being able to listen properly when playing in a band. It's important that you listen and try to hear all of the things going on. Are you in time? Are you in tune? Are you too loud, too quiet? The list goes on and on. The same goes for playing live. Are you listening to the drummer? The bassplayer? When you listen properly, you make continual adjustments that makes your performance that much better. You play in time, you are sensitive to the overall dynamic of the band and the song, and the band will just sound better. Everytime you pick up your instrument, make sure your ears are wide open.
Memory
I've written about this in a past post but it needs to be stated again. One thing that musicians must have is a great memory. It's important to remember all of your chords, scales, melodies, licks, fingerings etc and know them like the back of your hand. Charlie Parker was famous for the fact that he could remember and play back hundreds of licks, scales and melodies in all keys. How much of this is entirely your memory? The same goes for writing. Once you've memorized ideas, it's all too easy for them to 'pop up magically' in your songs. It comes from being familiar with the style but that really comes down to the material being internalized and memorized. That's why it's important to write something that you're familiar with because you've more likely memorized many facets of the style without really realizing it.
Consistency
One of the things that musicians are known for is their consistent practice at their art. One of the best and most effective ways to learn and master anything is through consistency. It's important that not only you practice everyday, but you're consistent in that practice. Practicing one thing one day then trying something completely different the next without coming back to original may be fun but it isn't very productive. The best way to internalize ideas, get your muscle memory working and mastering your instrument is practice the same fundamentals consistently.
Diligence
This follows with the consistency factor. It's important that as a musician you have a certain amount of diligence when it comes to learning the craft and especially when trying to achieve some success. It's true that being a musician isn't an easy way to go and you'll need diligence to make it through the rough spots. Other things, like sticking to your practice regimen, practicing stuff that you don't find all that exciting and trying to get something done everyday without much support also come under this topic.
Initiative
One of the other important traits to have is initiative. It's one of the things that's drilled into us since we started out first practice sessions. In fact one of the things that regular practice teaches us is to have the initiative to work on our own and try to keep motivated. It's not just the practice room that needs initiative either. You're going to need to get most things started and keep them going on your own. There is some support in the music industry but not much. You're pretty much going to have to figure most of this stuff out on your own. That includes everything from how to get a gig, to how that next verse is going to go.
Love of the Art
If you want to do this for a living, you're going to have to love it. You're going to have to love it just for what it is. You're going to have to love it, pursue it and try to get better everyday for no other reason other than the fact that you love to do it. Music is just too tough a career choice for anyone who isn't right into it. Even people who work in the music industry, who are in supporting roles have this attitude. Second, it's this love that will push you to do all of the things that you're going to have to do to become great at your art. It's a long journey and there needs to be that internal motivator for you to push through and become a great artist.