What do you wish you knew you before you bought your first nice bike?
Prioritize more: 


Tires -- the bike must allow me to use the tires I want to use.  I used to have a road bike that did not allow any tire wider than 25mm.  Any bike that cannot use 35mm tires is useless to me.  Some people are still under the illusion that narrow tires are fast, wide tires are slow; there is a bunch of research that indicates this is not true for tires up to 40mm, and at the relatively low speeds most people ride (&lt;25mph).  Wide tires allow you to ride on all surfaces without worrying about gravel, bumps, railroad tracks, bridge grates etc.
Fenders -- if it's too hard to install decent fenders, I am going to hate riding when it's wet.
Gearing -- if your highest gear is a bit too low, you're not giving up much. In the rare case (downhill+tailwind) where you could use a 53x11 gear for a few seconds, you'll just go a bit slower.  If your lowest gear is too high, there are some hills you can't climb, and other hills that you can climb only with great difficulty. To me, this suggests that overall lower gearing works better.


Prioritize less: 


Weight -- if a bike weighs 0.5 pounds less, but is uncomfortable on longer rides, that really limits my enjoyment.
Pretending to be a racer -- unless you are actually going to use the bike for racing, resist the temptation to pretend you're a racer and need the lightest wheels and the shortest wheelbase.  In practice, this is not useful for most people (heavy riders end up damaging superlight wheels, etc.)  Before you decide that compact cranks are for wussies and get a 53/39 crank, think about how much you're really going to use that 53/11 top gear. How fast would you have to go for that gear to be useful ?


Surprisingly like:


Internal cable routing -- I can attach a Revelate "Gas Tank" bag to the top tube, put it on any kind of bike carrier on the car, and easily carry it on my shoulder without cables on the top tube getting in the way