It's mainly the tyres and the weight.
My mountain bike only weighs 22 pounds and I have a second set of very, very light wheels with 1" slicks on them. The problem with road bikes is that they are massively uncomfortable compared to a mountain bike that fits you well, and road bikes aren't ideal for commuting. But they are quick.
( the mighty badger Aphrodite, on a bar stool, by your side, Fri 8 Apr 2011, 9:54, 2 replies, latest was 15 years ago)

Commuting is 90% of what I use my bike for
I'd rather have a bit of extra weight/resistance and it take a little longer than spend less time cycling but uncomfortable.
(berk, Fri 8 Apr 2011, 9:59, Reply)

I was only uncomfortable for the first week
There's really nothing to it now
( Peej, Fri 8 Apr 2011, 10:03, Reply)

so's being buggered by "Dutch Schteve", mate.
Doesn't make it good.
( the mighty badger Aphrodite, on a bar stool, by your side, Fri 8 Apr 2011, 10:06, Reply)

I totally agree that road bikes are massively gay
and I love my mountain bike and my bmx and ride them around town. I got the road bike because it was costing way too much to drive to work and I tried it on the mountain bike and it took forever. Then it was all downhill, I was wearing shirts and board shorts and the shirts were just disintegrating and the shorts at anything over 30mph where like riding wearing a kite so I got proper gear and look like a massive bender.
( Peej, Fri 8 Apr 2011, 10:13, Reply)

if you ever find yourself putting postcards in the spokes
its time to start counselling
( zulu eh?, Fri 8 Apr 2011, 10:19, Reply)

depends on the commute
mine is 15 miles of beautiful country lanes I even get to ride past this gorgeous lake every day. Its a bugger in the winter though, can't see a thing.
( Peej, Fri 8 Apr 2011, 10:02, Reply)

road bikes destroy my back.
They are also unusable on my commute (unpaved canal towpath) and the wheels have to be rebuilt every other week because of the roads up here. But it's totally horses for courses, I agree.
( the mighty badger Aphrodite, on a bar stool, by your side, Fri 8 Apr 2011, 10:07, Reply)

the drop bars on my road bike are a little low
i spend most of the time with my hands on the upper bit of the bar so think its flats for me... otherwise you're right, it just kills your back
( zulu eh?, Fri 8 Apr 2011, 10:11, Reply)

yeah, that's my point
if you're going to ride a road bike sitting upright .. you might as well ride a mountain bike.
( the mighty badger Aphrodite, on a bar stool, by your side, Fri 8 Apr 2011, 10:14, Reply)

i've set it up as a singlespeed hack, so for commuting its perfect
its a cyclecross bike so a little tougher than a normal road bike and it means i dont have to lock the expensive mountainbike up outside of the office...
( zulu eh?, Fri 8 Apr 2011, 10:17, Reply)

good call
I built a bike to commute in London out of a cheap kid's bike frame, handlebar for a seatpost to get the length right, singlespeed with a freewheel brake and spuds. Was genius. Until the seatpost/handlebar broke. Ow. And in the end someone STILL nicked it. Cunts will steal anything.
( the mighty badger Aphrodite, on a bar stool, by your side, Fri 8 Apr 2011, 10:24, Reply)