I just bought this frame for a build up project

https://tulsa.craigslist.org/bik/1045250200.html

After looking I got to wondering....how much difference is there really in the geometry of a MTB frame versus a road frame. Besides thicker tubes and obvious fork difference is there really that much?

The thickness of the tubing is not a factor in "geometry", but the fork rake and trail are probably the single biggest factor. Then there's the overall wheelbase and chainstay length, probably quite a bit longer on the MTB (keeping in mind the inherent differences between bikes for 26 in wheels and 700c wheels).

It's hard to tell from a picture, but 1980's era mtn bikes like that one sometimes had the almost exact kind of frame geometry as would make nice touring bike, so I'm guessing you could work with that frame.

A lot of people in the touring forum recommend non-suspension mt bikes from that era as a good, cheap touring bikes. So if the geometry resembles touring geometry, then you're looking at slack geometry and long chainstays. Likely too slow for road racing and a bit heavy and slow for a century bike, but would likely work very well for touring, winter bike, utility bike or around town riding.

Edit to add: just noticed it has double eyelets in rear, can't see the front, but that would obviously play well for touring or grocery bike. Vertical rear dropouts also make it easier to get the wheel on when fenders are installed.

I use non suspension mtb style road hybrid. I use it for touring, road riding, and for light trail riding. I ride with 32's and can go down as far as 23's if I get new wheels. I wouldn't really know whether to call my bike mtb or road.

I would call that a flat barred road bike. Hybrids I consider to be comfort bikes with a suspension fork/seat post, fat 700c wheeled, big fat cushy saddle, and adjustable stem w/ riser bars.

The FCR is a "performance hybrid"

soonerbills, your MTB is pretty much the perfect touring rig if the fit is right. I wanted to use an MTB to build my girlfriend a touring bike, but the biggest issue with MTB conversions is top tube length. Now if you are not using a drop bar it's fine but typically MTBs have longer top tubes which makes conversion to drop bars less than ideal as you typically would need a super short stem to facilitate the proper reach and therefore get funky handling. In the end I couldn't find the right frame for her short reach needs and settled on the LHT which pretty much is an MTB in the smaller sizes.

MTB frames tend to have a longer wheelbase and higher bottom bracket than road frames, and slacker head and seat tube angles.

The 'mtb styled' hybrids with 700c wheels like X posted are undervalued imo. They aren't cool enough to get hipster taxed so you can pick used ones up for 1/4 to 1/2 of what a pure roadbike goes for. At least in my local inflated market. If I were wanting to do a budget touring rig thats exactly what I'd be looking for, an old lugged rigid hybrid. Plenty of fender/tire room, and its a good bet they'll have cantis and lots of braze-ons. Pull the 38mm tires off and put your 32's or whatever on there, add your drops fenders racks and panniers and you're good to go. Barring that, yea a MTB with slicks and drops could work almost as well I'd guess.

Here is what it looked like fully loaded.