put over 1000 miles on a rigid MTB last year. Poor thing sits in the corner now that I have a road bike.
Problems with a mountain:
1. Gearing is for slow technical trails. My first touring bike had almost the same crank gearing of 22/32/44 (my mtb is 22/32/42) . I was running out of gear on the road. I upgraded to a 26/36/48. It is just the right compromise for the hills around here. I just recently set my original touring bike back up after finding the next size up frame and moving my 9speed STI's over to it. I find I really don't like the 22/32/44 for the street now. I still can run out of gear with the 48 but not nearly as often as I was with the 44. The center ring where I spend most of my time in the top 4 gears on the rear of the 36 seems just right and I can jump up to the 48 front when I get on a down hill. With the 32 I spent a lot more time jumping from the 44 to the 32 and just never finding the sweet spot on the rear. I would be in the top two on the rear all the time on the 32 center and having to shift up and down out of the 44 more often. I spend a lot more time shifting as a result.

2. Riding hand positions. You have 1 with a straight bar. It was killing my wrists on rides over 10 miles. I regularly do 30 mile rides now. The Hoods are where it is at most of the time but the drops to duck out of the wind on down hills and windy days and the tops to rest the wrists makes a BIG difference. I did run trekking bars on the Mountain for a while but found that my hands most of the time were on the sides or upper hoop away from the brakes and gears so I was all the time back and forth. That was a real negative when in a climb to have to remove your hand from the good power position and reach down near the stem to drop a gear. Felt very unbalanced and you backed out of the climb. WIth the STI I can stay in the power with both hands on the hoods and flick my fingers to down shift as my cadence drops on the climbs.

3. Drops for better aerodynamic. Makes a huge difference when fighting a head wind. Even with Trekking bars you still can't tuck in a comfortable position for very long like you can on a road bike. Now I ride touring bikes and the bars are up a lot higher then a "race" or "Sport" road bike so the drops are not severe but still a big improvement.

4. Tire sizes. My MTB was running A specilized "CrossRoad" on the rear rated 85psi and actually had a small on the road foot print with the locked center tread and high tire pressure. I ran a Specialized Hempsphere front that again rode up nicely on the center. It was a really good combination on the road but it still didn't fix the fact that I was spinning a LOT of tire. It takes effort to keep heavy tires spun up at road speed. I split the difference on the Touring bike. I run 700x35's (though I am eyeing a 700x32 as my next tire). Enough tire for the occasion off pavement excursion. Enough tire to take the harshness out of the road that a 700x23 has but considerably lighter then the 26x1.95's. The difference in speed was dramatic.

I have three good hills and my best time ever on the mountain was just over 18minutes and I was pushing. On the touring bike I made my ride 15:42 seconds Friday and felt less wore out and really didn't feel I was "Pushing it". I have made the ride in just a hair over 15 on a crit bike with 700x23's. Again not pushing hard but I did keep a good pace. Between the tires, wheel size and gearing it was just not easy to make good time on the Mountain bike. The mountain on this one good down hill I felt I was doing good to hit 32mph peddling as hard as I could. I regularly tickle 40mph and really putting the hammer on I have managed 42mph.