I too would like to know what to look for when buying muslin.In a rush to make a purchase before the store closed for the day I grabbed a bolt of 108" muslin thinking I would use that to back some large quilts and the feathers I just learned would show nicely on it. I just cut my first piece and pinned it to the frame - I can see daylight through it!
The label doesn't have any thread count on it. I now have a bolt of muslin & don't know what I can use it for.