» What is the most important aspect of playing solid poker?

Realtors talk about "Location, location, location."  Winning poker players talk about "Position, position, position."  Poker is a game of incomplete information.  The closer you are to the button, the more people must act before you.  You therefore know what moves those players have made, while having fewer players behind you to worry about.  Typically, you can play a wider range of hands from later position.