Damn, Invictus always beats me to it
Let's see, from most important to least important:
Invictus has it right on, you need a docile enough snake that can withstand newbie mistakes. So with that said, Ball Pythons are shy but also sensitive. But on the other hand, you don't want a GTP or something that will be musking and flailing around. So definately a combo of docile but hardy.
Well, to me anything up to five feet is a good one. But honestly, I've never thought size is the most important factor. People have had their first snake as a BCI or a Burmese, and have actually done very well. For a beginner who is also young though, five feet and under I would say. Like full grown corns and antaresia.
I personally don't think that a first timer should be spending thousands of dollars on a Kahl Strain Albino Boa. However, I also don't think they should be getting like WC Mite Infested "evil" pet store cheapy snakes. So I think they should get a snake from a good breeder but not go over board. So probably in the 100-300$ range.
4. Care Requirements:
Care requirements should be an important factor. Especially for cage sizes. If the beginner isn't educated and buys a baby ball with a 15 gallon tank thinking it'll be in it for the rest of its life, bad bad bad. However, I don't think the care requirements are important when CHOOSING a snake, I think it's important for a first time owner to be EDUCATED in the proper care requirements.
5. Life Span:
They should purchase a young snake so that they have a long time with them. Like 15 years or so.
Not really that important seeing as how there are SO MANY color variations and different snakes to fit this description. For people who like Pythons and Natural colors, again, go to the Antaresia grouping. For people who like a splash of color, I'm sure they'll find a corn to fit their liking.