Weight is the most important thing. A lighter bike will definitely be faster, and it could be more pleasant, because a light bike rewards you for increasing your effort. The way it responds to hard pedaling is less body-punishing than a heavy bike. I believe that one reason heavy bikes are slow is obviously the efficiency of converting power to motion. The other is the fact that each bike has a different cutoff point where adding effort becomes unpleasant. That cutoff point is higher with a lightweight bike than with a heavy bike.

Road bike encompasses road racing, cyclo-cross, sport-touring, touring, and various other designs. 700c wheels are almost always the size now. Lightweight wheels can go to astonishingly high prices, so the sky is the limit, but the more you spend, the lighter they'll be. If you ride with good form, it's possible to get long life out of any wheel set, even if you buy high-end racing wheels. This is if you don't put a lot of cargo weight on the bike. Believe it or not, a lightweight racing bike is perfectly suitable for people who don't carry much and like the way it rides.

Also, your choice of tire is even more important. If you get light tires and learn how to avoid flats, you'll fall in love with them. Tires are worth whatever you spend, and it pays to buy nice tires.