Great artists develop with time and passion. However, you also need to grow comfortable with your new art medium. You can't really begin to devote yourself to whatever art you are working on until you are familiar with the medium.
To take a non visual arts example, a musician cannot truly master and enjoy creating and playing music if they are still uncomfortable with basics like finger positions and such.
I know from the experiences of others and myself that digital art is difficult to transition into, if you've only done more traditional art before. Not only do you need to get used to the tablet you are using, but you also need to grow familiar with whatever program you are drawing in. SAI, photoshop, GIMP... It takes time to grow accustomed to the details in each program. --- Don't get frustrated with yourself. Keep on practicing and improving yourself. And if you need anything, deviantart is a fantastic place to get advice, critiques, etc. Most artists here would be glad to help those who ask for it.
Don't get discouraged, be inspired if you look up to their work. Remember, today's great artists, they all started out like you. They worked extremely hard to get to where they are now. It all depends on how much you want it. I personally know one secret of them.
Do the thing you love to do the most! And own it.
Blow people away with each artwork you make. Give them the feeling that someday you'll be as great as the artists you look up to. Be strong in what you believe, then you'll get far : )
When you can not think of anything to draw, work on your problem areas. If you are bad with hands, get a bunch of refrence pic's of hands and go to town. If you are bad at buildings, do some 1, 2 and 3 point perspective work. How the great artists got that way was starting simple, and building up. BTW you have a bunch of anime/manga artists there, and they usually start with realism, then move to cartoon styles. And if you believe you can, or believe you can not, you are right. So as long as you are sitting there saying "I can never be that good" you are right, and if you are working on your skills, saying "with enough practice, critique and effort I can be as good as them" you are right.