If I were to replace my wheel set, what do you look for in an MTB wheelset?

I’m assuming a quality hub that can be rebuilt?

Good quality hubs and regular j-bend spokes. That way if you break one you can go to your LBS and get a new spoke that day, not wait days or maybe weeks for those fancy ones.

I second that. Easy to fix, rebuild and replace parts.

About hubs you want to consider serviceability too. High quality sealed cartridge are nice and low maintenance but may require proprietary tools, which could mean a trip to the LBS or worse return to manufacturer for service. But it may be a long time before they ever need any service at all. High quality loose ball hubs if properly maintained last and perform just as well in my experience. I rebuild all the hubs in my house at minimum once a year but some get done much more often, just depends on the distances and conditions they are subject to.

If you are planning on running tubeless there are a couple important things to watch for. Make sure the rims have the ridge that holds the tire out against the bead. Even some ‘tubeless ready’ (or tubeless whatever) rims do NOT have these. Obviously UST rims have the ridge, and there are some others, who don’t want to pay for the UST designation, that have the ridge too. Now make sure that the tires you are using have a right angle on the inside of the bead so the ridge holds the tire on the bead. Watch some UST videos to see what I mean. Without the ridge you run the risk of burping air from the tire under high side loads…or worse, pushing the tire right off the bead and causing rapid deflation. I would also recommend going nice and wide with your new rims, especially if you are going tubeless. Of course you’ll have to balance that with your weight tolerance. I am personally a big fan of DT Swiss hubs. I really like their ratchet drive mechanism. If you opt for a wheel system that has straight pull spokes, I would recommend purchasing a few to have them on hand in case you break one. Perhaps you should check with your local bike shop to see what they service and stock. You may also want to consider acquiring the parts individually and having a local wheel builder finish them for you. Have fun and do lots of research!