1.Simply the ability to create aesthetic draw/appeal with his music.
2. Often difficulty in the playing of music makes it more prestigious and impressive when performed. This can add to to a composer's greatness
3.The ability to write something that someone else not only didn't come up with first, but possibly could not have ever come up with first. I don't think that anyone but Wagner could have written the beginning to Tristan and Isolde with that specific melodic and harmonic element and the famous chord from that section. And it doesn't sounds like crap. Its actually pleasing to listen to.
4. The ability to write with an agenda/ idea/ picture in mind and portray that. Not just the ability to write music, but to put purpose/ meaning/ whit/ emotion in the music. I'm not just thinking of program music that tells a story here. I've always been really impressed with Schumann's ability to compose not only good sounding music, but to also employ humor or irony in his work. Or what about Schubert's ability to compose some aspect of nature into some of his lieder: like hoofbeats, or fire, or water- those things are just the background extra stuff behind the song and I still think that they are so impressive.