Quite a few threads on the subject and it is mostly a personal preference issue.
For the distance you are traveling the main things that are going apply is most return on your effort. Tires are the biggest factor followed by gearing.
For the most part a light weight wheel combination is the biggest return for a commute of 15 miles. The heavier the wheel assembly the more effort it takes to get up to speed and maintain speed. You need to also concider the roads you will be riding. a 700x23 while being a fast size is also going to transmit a LOT of road noise to you and the bike that will wear on you. I have quite a few bikes and 700x28-700x32 (about the same as a 27x 1 1/4) is the sizes I find I like the best for commuting. The bigger tires tends to soften the ride without a huge impact on effort. You look at touring bikes made for long distance and most are running 700x32-700x35. The road surfaces you ride are goign to play into this a lot. Bad road or unpaved...wider is better.
Next is gearing. If the area you live is flat you need less range then if you are in a hilly area. The heavier you and the bike are the more this becomes important. I live in a relatively hilly area and for commuting I really like a 48-36-28 crank set (common to touring bikes) with a 11-28 cassette. I sometimes will spin out on the top end but there is no hill I cant conquer.
Mountain bikes with front suspension can make a good city bike with a set of slicks but they are typically gear for climbing hills and low speed so you will spin out frequently with the stock cranks (42 or lower big rings) on the flats.
What makes a bike a Hybrid is the gearing. It is close to a Mountain in appearance but it is gear more like touring bike. The speed is there when you need it.
Rear Suspension or not....Suspension may smooth out a road but you usually looses some forward propulsion efficiency on bikes with rear suspension. If you commute had a significat portion of unpaved..maybe it would work otherwise a ridged tail is the most desirable.
Front suspension. Hurts a little in forward propulsion. Bad or unpaved or jumping curbs in the city it is a good trade off except for weight and potential for extra maintenance.
Straight bar, Trekking bar, Mustach or drops. Long distance the straight bars may be a problem. You have one hand position and you may have discomfort after 4-5 miles. Trekking bars on a upright bike (mountain/Hybrid) solve the complaint.
Drop bars big advantage is aerodynamic. Riding a 15mph average most people are in the hoods or tops. You start running into a 20mph head wind your effort goes WAY up. Moving into the drops cleans up your aerodynamic profile in that instance. An option you just don't have on other styles of bars.
I like touring bikes. Gearing is good for the flats. I can run larger tires for a better ride, Drops when I need them, mounting points for racks to haul junk on.
Last edited by Grim; 07-25-10 at 11:54 AM.