I was thinking about that today and how I should have used fewer slides at RubyConf. The problem, in my case, is a question of mechanics. I was trying to use slides, then jump to Eclipse, then jump to the Services GUI, then back to the slides, etc. It didn't flow that well because I kept having to minimize one program and maximize another.
There was also the problem of having way too many slides. Better than too few, I suppose.
Next time I'm going to reduce most of the slides to notecards that I can refer to if needed. I'm hoping that the talk will flow a little more smoothly that way, as there will be less pointy, clicky going on. This is, I think, a better approach when your presentation contains a fair amount of interaction, rather than just informational material.
I suppose there's the drawback of not having any slides available for others to peruse afterwards. I'm not too sympathetic, however. If you wanna know what I talk about, you have to come to the conference.