Materials:
I have a preference for leather unless I am going to be in the water or soaking wet. There are a few places I like to hike with a lot of stream crossing (sometimes the hike is up a stream) -for those hikes I like synthetic sheaths. As long as the synthetic is tough it is fine with me.
Style: -for regular sized bush knives - blades of about 3.5"-5"
I prefer a deep carry sheath with a swivel or dangler attachment. A loop is okay as long as it is high on the sheath. I also like scandinavian knives that have a pommel stud and a keeper strap for that stud built into the sheath. When the knife is sheathed it is comfortable to have as little of the handle as possible coming up above my belt line. I don't like when the handle of the knife sticks up above my belt -that really annoys me and I have never found it comfortable or useful to have that high.
Also, if the sheath is not a deep carrying sheath, then it had better have a really good retention strap that straps around a logical point of retention (ie -not a strap that goes around the middle of a handle -allowing the knife to move upwards until the guard finally catches on the retention strap, and by that time half of the blade is exposed for your fingers to come in contact with as you try to re-secure the knife -yah, I really don't like that). Kydex retention is okay but it has to be followed by a low carry attachment for my belt so that the handle is not sticking up high over my belt line.
I am not a fan of having pockets on my sheath, or loops for fire steels. I like to keep my kit gear separate from my belt knife. I like a sleek and simple sheath that is tough.
I have never been a fan of Tek-Loks or MOLLE Locks. I just don't have faith in them. They seem way to easy to disengage and I am not risking my most important tool when in the brush. Sorry, I know there are a lot of fans.
I do like to have grommet attachments -especially with kydex since it lends to so manny ways of attaching a knife using cordage. On leather, I don't like to have extra grommets unless the knife is very large (like a machete) -just a couple at the bottom is fine.
If the sheath has a loop -the loop should be the highest part of the sheath -I dont like when a loop is in the middle of the sheath (or top 3/4).
That covers most of my preferences. For really small knives I like synthetics -like an Izula etc. For really large knives and machetes -I generally strap them to my pack. I would like to try a thigh sheathing system. I bet that it would be comfortable -especially for larger knives.
GOOD THREAD -sheaths are a big factor in finding a knife. It is always great when your favorite knife comes with your favorite style of sheath.