For me it was Termination Shock that finally convinced me to stop reading his books. He just likes to write really long, repetitive and wildly overly detailed books. I was entertained by SevenEves and Reamde but I'm open to the possibility that I might very well react as I did to Termination Shock if I tried rereading them.

Edit: I've read and very much enjoyed a ton of Stephenson (Cryptonomicon, The Baroque Cycle, Anathem) but his recent stuff is tailing off for me. I don't know if it's me or him.