Mountain bikes are great for touring. I would recommend getting bar ends to give your hands another position on the handlebars. It's slower than a touring bike though because of the tire resistance and more upright riding position but you will get a smoother ride with the larger tires. I've noticed a lot less broken spokes on MTB's touring vs hybrids or thin tire touring bikes according to journals I have read. I wouldn't recommend a full suspension MTB for touring the road, get a hardtail or one with no shocks at all.
I definitely had a better time on my $20 good-will mnt.bike doing the Natchez Trace and then down thru North Carolina and to Georgia. I had over 50 lbs in the bob and another 30 in bags up front. The gearing was spot on for me.
MTBs have some advantages for touring. The frames are generally heavy duty and can easily stand up to heavy loads. The standard gearing is lower than the typical road bike so you don't have to cutomize it. 26 inch tires and tubes are easier to find in small towns than 700c.
The front suspension on a hardtail can also be nice for long days on rough roads though it does add some weight to the bike.