The major problems I had initially with ie6 and previous versions was padding, things have improved a good bit but there are still issues between both... most experienced html / css developers simply avoid the problems naturally.
What I generally do from a padding point of view is create an outer div of specified size, then create an inner div which will expand to 100% of the outer div and give that a padding, that way the content inside the inner div will pad naturally and consistently on most browsers.
The reason you must do something like this is because when you pad a div in ie it makes the div bigger where as in firefox and other browsers it shrinks the content of the div... so an 800px width div with padding of 20px would be 840px on ie, which can mess up formatting etc.
I know it's probably frowned upon but sometimes I still utilise tables for elements of a site because of their consistency between browser versions.