Hobie Cat has a very successful small rotomolded catamaran out and Sunfish Laser has introduced the first rotomolded monohull. There are many problems with trying to build a monohull out of plastic using the same design that you could for fiberglass, but with innovative new designs like the Escape it could prove to be the material of the future for small boats. Your sailing skills are a factor to consider. You don't want to get a boat that is way too complicated or tippy if you are just learning, but you do want to be able to learn new techniques and skills if you have mastered the basics. Be sure to get a boat that you can feel comfortable sailing.
Capacity is a big factor.
If you want to be able to take the whole family out for an afternoon, you'll need a fairly big boat. If you are going to be spending most of the time sailing solo, you have more options. Some boats can be set up and ready to go off a trailer in a matter of minutes, others may take a hour or more. If you need to set your boat up each time you sail, the quicker boat to set up is going to be used more and you'll have more fun. If you plan on keeping your boat rigged for a long period of time, you don't need to worry as much about set-up time.