The bicycle I most often ride is ...

I really question whether you actually ride on the road. My experience is that the majority of mountain bike riders are bicycle-pedestrians that ride on the sidewalk without the slightest clue that it's often illegal.

Any argument that road bikes have less stopping power or likelihood of falling over due to pot holes is really silly so let me explain...

1. Mountain bikes have knobby tires which are designed to grab rocks and dirt and not provide traction when cornering on asphalt, cement, etc. As such, the likelihood of a slide on a mountain bike in a corner is vastly increased.

2. Road bikes generally use a 700c (centimeter) wheel which is about equivalent to a 29" mountain bike wheel; however, most mountain bikes sold at box stores have 26" wheels. As such, it's easier to go over a pothole with a road bike because the wheel is less likely to fall into the pothole. There's simply a larger surface to distribute into/over the pothole. The tires on road bikes are generally pumped to between 85 and 120 psi whereas the tires on mountain bikes are pumped to much lower levels. This means there is less deflection on a road bike and technically less likelihood of a pinch flat with properly inflated tires.

3. Brake distance is a factor between the amount braking prior to the sheer limits being reached. With a road bike tire there is more surface area in contact with the road at any given time which means the sheer limit is higher between the road and the tires. However, mountain bike brakes (cantilever brakes usually) are often capable of putting higher amounts of force on the rims which would cause the wheels to stop moving. If the wheels stop moving too quickly the sheer forces climb rapidly which may cause a loss of traction as the wheels reach their limit. In short, if you know how to properly brake with a road bike you're more likely to stop faster.

4. Road bikes are capable of much greater speed on roads (in general -- I've seen some really fast mtb riders before) and all the above factors really come into play when you're doing 35-50 mph on a bicycle and need to execute a fast turn or brake quickly. I'd never want to be cornering at 50mph on a mountain bike because the knobby tires would likely break traction or they'd introduce a death wobble.

Knobby, mountain bike style tires are actually terrible on the road. They corner very badly and have awful rolling resistance due to the wiggling of the knobs. Slick tires give better traction under both wet and dry conditions on pavement. I can take sharp corners in wet conditions on my racing bike way faster than cars can. Car tires only have tread in order to prevent hydroplaning, which is completely impossible on a bike (due to the narrow tires with an ellipsoid contact patch), unless you are going about 100 mph. So, if you do have a mountain bike, get slick tires for commuting.

The best option for commuting is actually a touring bike. Wide slick tires, steel frame, strong 36 spoke wheels. They are made to carry tons of stuff and have mounts for full fenders. All of that and they are 90% as fast as a road bike. Another option is a cyclocross bike, if you get slicks. Mountain bikes are a pretty poor choice for road riding. Knobby tires sap tons of energy, and corner very badly. Slick tires provide much better traction on pavement. If you have sensible wheels (28-36 spoke, not 20 spoke gimmicky racing wheels) with 32mm+ tires, potholes are no big deal. That said, I ride about 250 miles a week on often awful roads, with an aluminum frame and 23mm tires, and I do just fine.

Most bikes marketed as commuting bikes, like "hybrids," suck. They offer the same limited hand positions as a mountain bike, and usually have heavy drainpipe aluminum frames (but it must be light because it's aluminum, right?). Drop bars on a road or touring bike look scary, but they give you so many more hand positions, and more ergonomic ones. You have the flat bartops, just like on a mountain bike, as well as hoods and drops.

Unless you are racing, get a quality steel frame. Touring steel frames can weigh under 2kg. Lighter steel frames are made under 1500 grams, which isn't that much different than a 1 kg fragile and ugly carbon frame (Quality aluminum frames aren't made anymore). Get that, a decent groupset, and quality HANDBUILT (important!!!) high spoke-count wheels, and you will have a durable bike that will last forever.

On the subject of wheels, stock wheels on bikes always suck. They are machine built, and the spokes normally have widely varying tension. Some will be undertensioned, which mean they are prone to going slack when you hit bumps, which causes metal fatigue and eventual spoke breakage. Wheels are prestressed structures. The weight is supported by the bottom spokes as rigid columns. The hub does not hang from the top spokes. A better option is to get handbuilt wheels. A good set of handbuilts with Shimano Tiagra hubs (quality, but not expensive) and Open Pro or Velocity rims will cost around $250, which is at least as cheap as your average factor wheel. Or, you can build it yourself if you have some patience and access to a truing stand (it's fun). Another added bonus is that when the rim wears out from braking, you just have to replace the rim, not the whole wheel.