What should I consider when choosing a bike frame? In thinking about building a road bike from scratch, what should I consider when choosing a frame?

I'll be building from all new components and plan on just doing day rides of a few hours a piece, but perhaps few days touring.  I'm not really interested in racing at all—that's what cars are for :-)
I put together a bike from parts bought on eBay as well as locally, and it was very successful.  I was lucky enough to get a frame that fit me well; however, as it was a 1998 aluminum road racing frame (Cannondale CAAD 3), it is really, really stiff.  For riding just an hour or two that doesn't really matter so much, but longer rides are fatiguing due to the construction and materials.  Recently I rented a high-dollar Specialized Roubaix, and couldn't believe how much smoother the ride was.

Materials


Aluminum:  stiffest there is
Titanium:  reportedly a nice ride, but very pricey
Steel:  high-dollar frames will be very nice, forgiving; cheap steel frames will feel like you are riding on plumbing pipe, dead and heavy
Carbon:  expensive, although coming down in price; smooth ride yet agile feeling
Aluminum with Carbon fork and/or stays:  more carbon means more forgiving, smoother ride, more $$$


Geometry


Road race bikes:  stiffer geometry, more responsive, more fatigue; definitely the most common on the market
Touring bikes:  more relaxed seat &amp; head tube angles, longer wheelbase, room for fenders, eyelets for mounting racks; harder to find
Triathon/Time Trial bikes:  DO NOT GET unless you truly plan to do a tri-  very upright geometry based on leaning forward onto TT bars, harder riding (unless you spend lot$ on carbon components)


Depending on how much you plan to spend, it may be worth it to pay about $60 to get a professional bike fitting.  The right fitter will find out about you, your goals, and your flexibilty to come up with the right shape for you.  If you have spent a lot of your recent years at a desk you may not have the flexibility to lean as much as the average frame wants you to do.  Also, different brands may fit you better or worse.  When Lemond bikes first came out, they seemed to fit people best who had longer torsos or lots of flexibility.