It was mostly the need to zoom and scale. SF is heavy enough and the 3G is slow enough that zooming and scaling was not a quick process, and in threads in which images featured (which, given the nature of this forum, is a lot of them!), the images would often force the text block width to a size at which text was unreadable. To read it you'd need to reach an arbitrary zoom level and then scrub left and right as you progressed through sentences, which was just beyond tedious.
This is not the case on the iPhone 4, but at this point I just like poking at the codebase when I'm bored.