It's called a Nash Equilibrium. There's little that either party can do, because they know everything that each other can do. Vader can predict what Obi-Wan will do and exploit it, and Obi-Wan knows that he'll exploit it, so he doesn't do anything.

i will agree.
unfortunately, i am 'that' anon. the one who MUST correct you like the grammar nazi's.

basically a quick wiki scan says that Lucas used Kendo styled sword fighting to try and convey that this was essentially "two old men fighting"
heck i reckon Vader held back as well. i'm just going to say that given all the servo's etc. in his limbs he could've floored Ben in 3 seconds tops.