For me, the problem is twofold. First, when I watch a slide show, I invariably slip into "television mode." I sit in my seat, utterly inert, watching the pretty transitions and custom bullet points dance across the screen. All the while I am not internalizing a single syllable of what's being said. Keynote slideshows are even better for zoning out, as eye candy is such a major part of the application.
Secondly, I sometimes find myself critiquing the slideshows. While I hate watching these things, I've been forced to make several for different departments at work. Similar to the way a musician can't listen to a piece of music without analyzing the chord progression, I can't watch a slideshow without questioning the presenter's technique.
So, what's your take on slide shows? Does the thought of just one more professional conference make you cringe?