Personally, I'd like one with lots of partially developed stock material, something 5-10 years from being a great bonsai. For example: Field grown deciduous trunks, field grown pines ready for branch selection/apex development, plants grown on/through a tile, twisted and fattened shimpaku. Most of the nurseries I've been to seem to have lots of immature beginner material and lots of completed/almost-complete bonsai, with very little developed stock. I'm lucky enough to have one nearby that has a good bit of partially developed stock, but they like to pot the best ones up and charge more for them just because they're in a pot (that I usually don't like).
Secondly, you have to have a good variety of material, not just different species but also different sizes, different shapes, different stages of development. One place I went to appeared to have just bought shipments of Chinese quince, trident maple, Japanese maple, elm and each batch had only trees of the same age that were all grown in the same fashion. There was very little variety within each batch. You could get a wavy trunk quince, a trident with movement but no taper, a zigzag JM, or a broom elm. If you don't grow all of your material yourself, I would recommend getting a few shipments of the same species from different vendors.
That's all I have time for right now, but I think I'll have more later.