My tuppence:
To add to what other people have said, DEFINITELY a bike with drop handlebars (i.e. racer type rather than flat handlebars). For those kinds of distances, you'll want to change your hand positions regularly, and being able to move down to the drops in a headwind when you feel confident enough too (which won't take long!) is a must. I got a flat barred bike when I started commuting and I REALLY regret it. Currently have a bizarre bullhorn based configuration which is an improvement, but my next bike is definitely going to be drop bar based.
So reiterate what others have said DROPS DROPS DROPS DROPS DROPS!
Now you have drop bars, this doesn't necessarily mean pure racing road bike. There are a lot of variations on themes. You have:
- Road bike - minimalist set up, low and fast, skinny-as tyres
- Audax - slightly beefier for harsher rides and able to take mudguards
- Tourer - longer, bigger, heavier, more upright for long load carrying journeys
- Cyclocross - beefier and heavier with chunky tyres for off road racing.
Then you have all sorts of 'hybrids' that mix elements of the above along with elements of mountain bike, utility, shopping basket etc type bikes.
Example - this is the bike I feel I should have got instead of the one I did:
http://www.cyclestore.co.uk/productDetails.asp?productID=22716
This is technically a hybrid - mountain bike frame and forks but based around thinner and bigger 700c wheels. Handlebar position is somewhat higher than a regular road bike for comfort and visibility (although I agree visibility is hardly reduced by riding lower). Wheel rims are flexible enough to take chunky 37mm commuter and offroad tyres, as well as narrower 25mm slick road tyres so you can swap depending on what ride you are doing. So it's very flexible and can keep up with the road boys if you feel like pushing it!