1. Fiber content: cotton , polyester, cotton/ poly, silk, wool , bamboo and others. (Consider allergies)
2. Is the batting recommended for hand or machine quilting. The later should be broken down into long arm and short arm machines. Some will be marked on the packaging some won't.
3. Shrinkage: most are labeled 1% or 3% etc. In a 108" quilt 3% shrinkage would make your quilt 104.75" Does it make a difference if they are air dried versus heat dried in a dryer.It pays to check this information out ahead of time.
4. How far apart can you quilt . Most will give this information in a maximum # of inches .
5. Does the manufacturer recommend preshrinking?
6. Is your project made of all dark colored fabrics. Do you want a dark colored batting? I know they can be harder to find , but they do exist.
There are questions about thickness and the end result you want . Such as do you want the look of an antique quilt ?
This is just food for thought. Don't be afraid to ask. If you like a project you see , ask about what product they used. As you can see there is more to batting that you thought. Hopefully this will help a little and let me know what else you think is important.