In order for an artistic piece to be great, it must e colourful and portray something that evokes emotion in the responder. It also has to make great use of lines and vectors. This of course varies within time and culture as different eras and different cultures have different ideas of art and have different ideas of what is considered as "great"
This is only based on opinion not straight facts: what makes an art piece great will vary between people, but for it to be great overall it should portray an image and show a theme in a persons mind and it should be liked by a variety of people.
