1) Large work surface with no obstructions:  No hutch, no little integrated file areas, etc.
2) Unobstructed leg room.  Nothing is worse than those old wooden desks where your legs are confined to a tiny little area.  I like to sprawl out.  
3) Sturdy construction, doesn't wobble, etc.  
That's it for me.  I basically just want a nice table.  If I need a file cabinet, I'll add one.  If I have other storage needs, I'll add them.  I hate desks that try to integrate everything all into one, and I especially hate desks that have keyboard trays.