I look at alot of portfolios, and have met models with poor portfolios, and models with highly professional portfolios, and everyone in between. While I am in no position to speak for the industry at large, I can certainly offer some ideas on what I look for when looking at portfolio.
It's an overworked topic, but one that still comes up on model and photography forums. There's always someone starting out, and wondering what should a portfolio contain? One of the reasons I make a point to always meet a model before scheduling work is to review not only experience, but portfolio. A great many models, surprisingly, arrive with no portfolio at all. I don't know how proper or improper that is, or if I should be more specific, but it is nice to see a few shots. When I do get the chance, I look first and foremost for a professional presentation. I'm not averse to working with new models, and a sparse portfolio should be no reason to avoid a good presentation.
As far as what should be in the portfolio, well, it really depends on past work. For models with significant experience, the ones I probably can't afford to hire, I look for samples that are similar to the work I'm hiring for. Looking at my portfolio online, or reading my project description, hopefully, is enough to assemble some shots appropriate for artistic work.
For models with little experience, anything professionally shot is good. And it doesn't need to be alot. We all start somewhere, and the best place to start is with a serious dedication to your craft. Which, for a model I guess, means building a portfolio TFP, on his own dime, or in school. One of my favorite models, who I invite every time I shoot, had, if I remember, only two shots in her book when we met. But they were two appropriate and well done examples, and it worked out well.
Specifically, if I could have a wish list of what I might find in a portfolio, in every one I would see a professional headshot. To me, the headshot is the navy blazer of the portfolio. The little black dress of the book. No wardrobe is complete without the blazer or dress, and no portfolio is complete without a headshot. While it's a given for actors to have one, it's nice to see a model who has invested in a professional headshot. (Disclaimer: My girlfriend is a professional photographer, has shot headshots, and I admit my partiality here to her influence.)
Another thing I'd hope to see is a full body shot. Unclothed or clothed is fine. Not all figurative work is nude, and as I work less and less with nudes I like to see how fabric will fall across a model's body. Either way, I'll be drawing the figure, photographing it first, so I like to see the figure on film. Give me a chance to imagine working with your shots, let me see an interesting form, or show me how you present yourself. I could just look at the model and see for myself, but I wonder if what I see in life is what I would see on film?
Finally, I like to see a sense of pride. I like to see a model with work she is proud of, work she considers well done. I like having the sense that a model will bring alot of positive energy to the shoot, which always results in better finished pieces.
It's not a scientific approach, and I'm certainly no expert on the modeling industry itself, but I have my particular approach, which may be individual, but not terribly unique. Hopefully, though, knowing a bit about how I work is a little more helpful information for those wondering "how do I start?"