What should I look for when choosing a kneeboard?
You don't want something so big that it interferes with fully moving your control yoke/stick.  I have one that is about 1/2 sheet of paper in size (or about the size of an approach chart), that works good for me. Decent width of velcro strap to go around your leg.
Sportys.com has Aluminim VFR and IFR kneeboards, with good info printed right on it.  If you roll your own, they also have the same info available as a placard you could stick on.  The kneeboard and a few pieces of paper are all I've used.
I fly my own plane, so I can leave pens/pencils in places that are convenient for me.  If you are switching between planes, then maybe a kneeboard with holders for a pen or two would be handy also.
I also have a yoke mount clip that holds an approach chart. I found that trying to read a chart on my leg under the yoke was too difficult.  I may change to an ipad holder on the yoke as that seems to be the trend now.