Money was (and still is) the biggest factor when I built my barn. It's pretty modest compared to a lot of you guys (28 x 48, 12' ceiling + loft), but I'm retired now and my needs and ambitions aren't what they used to be.
I built my barn as cheap as possible, so it's just sheathed in T1-11. Looks nice, but it's hard to properly add doors and windows. If I had the budget, I'd have used sheathing, then put siding over it.
I really wish I had put insulation under the slab. During warm, humid weather after a cold snap, it sweats like crazy. Turns out that that happens a lot in our SC climate.
I had originally planned to put a few courses of block above the slab, then wood framing. Too much money at the time, but now I wish I had done it.
I'm just finishing up the wiring. Adequate 120v, 240 V as needed, and a circuit of 3 phase from my rotary phase converter to run the machine shop tools. I finally put an outdoor outlet (I use a battery charger a lot on my old tractor), and power in the loft area for lighting and a lift powered by a 120V winch.
My loft is 28 X 32, with 8' headroom. It is strictly storage, and I built the floor deck stout enough to store a lot of **** up there. My goal for the rest of my days is to weed out the excess ****. Wish me luck! The lift is a 2 x 4' platform that lowers down through the loft floor. There is no guide system; I just have a rope tied to it to keep it aligned. It works fine, and takes up zero floor space downstairs when not in use. I only use it a few times a year when I'm putting stuff up there, or taking it down.
I'm eventually going to insulate and do interior sheathing; probably painted OSB. It will never be an energy-efficient building, but I'm hoping to be able to take the chill off. I have big sliding doors (12 x 12 opening), so it's pretty hard to seal it up. I could seal up the opening for the winter, but I park my daily driver (Miata) in the barn, so I need to be able to open and close the doors easily. I hadn't planned on that, but I'm not going to leave a well-preserved 27 y/o convertible out in the weather.
My lighting is cheap 4' LED units from ebay. They put out plenty of cheap light, but I need to figure out how to improve the light-to-light connections, as they keep going out. If I go up there and wiggle them, they work fine, so it's just the connectors. I need to take them down anyhow, as I need to insulate the ceiling, and then I'm going to put up white metal for a ceiling.