I am not an expert on felt, but I do know what I like. I've had several people ask me over the past several months what is so great about wool felt and if it really makes a difference. Since I started using wool felt I have a hard time using acrylic. Think of the difference between a wool sweater and an acrylic sweater. Same thing.
Here is a picture of 3 types of felt. From left to right: 20% wool-80% rayon blend, 100% wool, 100% acrylic. The wool blend and wool are denser. The darkness you see with the acrylic is the table beneath it. 
Acrylic is inexpensive and easy to find. Most craft stores sell acrylic felt. Sometimes you can buy 10 sheets for $1.
Wool blend costs more. I buy mine at Ben Franklin for 60 cents a sheet or $9.99 a yard.
Wool is the most expensive. Prices do vary, but I think $4 for a 9 x 18 inch sheet is the cheapest I've found. I've only bought from Magic Cabin and it's beautiful felt.
Wool and wool-blend felts stand up better to sewing as the fibers are stronger. This is acrylic on the left (dark pink) and 100% wool on the right. This is machine stitched. See how the acrylic fibers are pulling away from the stitching? The holes in the wool are just the needle holes. This might not be a big deal for some projects, but if you are making a stuffie you don't want this to happen as the stuffing will come through the seams and the felt can rip when pushing the stuffing in.
This is 100% wool (pink) and the wool-rayon blend (blue). The wool fibers help prevent tearing.
Acrylic felt is also very fuzzy and 100% wool is smooth.
You just have to get your hands on it to really know the difference. For flannel boards and other projects where I'm gluing I use acrylic felt. If I'm sewing I prefer 100% wool, but will use a blend if that's all I have.
If you have questions or opinions please chime in. A lot of people are curious about this topic.
Here are some sites to buy wool and wool-blend felt. You can also Google for other sites.
100% wool
Magic Cabin
A Child's Dream Come True
Kari Me Away
Blend
Marcus Brothers
Joggles (National NonWovens felt)
The Silver Penny
Vintage Vogue
**Edited to add: Go check out Alice's sewing felt tutorials: blanket stitch and whip stitch. And she is the queen of felt organization.
