This is a great question Trev, I would have to say the number one attraction is appearance but of course with this you face other areas which must be factored in. Most of my students will look at our 50/50 and say anything from "cool look at his colors" to "sweet his eyes are blue!"..... "look at him climbing the plant" The next is a question being "does he bite?"........."Can I touch him?". Observing the reaction from the kids, it seems as if a nice looking snake with a good temperment and personality is a must for a first snake. Don't forget he wouldn't be there if the going price for kings was $800 either, nor would he be there if he were to grow 15ft in size.
Myself, I have my heart set on a BRB and JCP after seeing them for the first time,
Therefore I am going to have to say for a first snake you need the following:
1. Appearance (obviously price and size factored in)
2. temperment & personality
3. housing requirements
4. Diet ( food readily available)