I just realised something about playing slide (though I've been doing it for a couple oof years now, I just never noticed). You have to be much more gently with your right hand. Obviously, it depends on what tone/style you're going for, but playing slide (particularly single note slide solos) is much more about "caressing" the note with your right hand, rather than simply whacking it with your pick.
Of course, once you get used to it and you feels more comfortable with that you can experiment with that great rough bluesy sound, but it helps to start light and really feel your way. It's a very expressive mode of playing, and you have to be very in touch with your... touch... :D