For snakes, I like an enclosure with plenty of floor space(the bigger the snake, the more space needed), good temp and humidity retention, a good sized front viewing area, and good security(i.e. locking door/escape proof) as well as affordibility and nice appearance. I too intend to make my snakes adult cages myself.

For arboreal lizards and snakes, height is most important, as is good ventilation and light. And for burrowing lizards and amphibians, a solid, sturdy, moisture proof floor is my first consideration.