Playing in tune and on time? Those are mechanical skills and can be learned by most. Just takes work and attention.
I think great playing is being able to communicate effectively with one's instrument. Of course, to communicate, one has to have something to say. Which is why most of us would rather listen to a great performance played through a tin can than a mediocre one on HDTV with surround sound.
So, to me, a great performer has the following characteristics:
- A reasonable facility on his instrument.
- The ability to communicate with his instrument
- Something to say
While most of us just see a squashed possum on the highway as just more yucky roadkill, there's someone gifted who will see it as an expression of a deeper message.
So, IMOHO, one can learn to be a very competent player with dedication and hard work, but to be the one-in-a-million true artist requires some sort of gift. I'm not sure that one is necessarily born with a gift, however--or if the gift is a result of some special experience.