Weather app fail this morning, dressed for the -1 not so much for the torrent of rain that accompanied it mid-ride. Trousers soaked but lucky I had my merino wool base layers on held up enough on the water but more importantly the temperature. All hail, premium sheep hair!
The primary difference is that Merino wool comes from Merino sheep. As for what makes the wool special, it appears to be the size of the fiber itself. The USDA has specifications for wool grades based on the size of the fiber, and Merino wool fibers range in size from <18 microns to 26 microns; for reference, a human hair ranges in size from about 50 - 100 microns. That puts all of Merino wool at grade 62 or better. Because it’s a finer fiber, each individual strand is more flexible, which makes it less scratchy. The flip side of that is that a lighter fiber means that it’s also less strong, so Merino isn’t well suited to outerwear that’s going to be used in more austere environments. Wool in general doesn’t have very good abrasion resistance, and a very light fiber will be much less so.
Any wool base layer made from sheep’s wool should keep up just fine, but a coarser wool is going to feel less comfortable against your skin. I have an old Army milsurp sweater, and the wool is very coarse; most people would not find it comfortable on their skin.