"Finally, why do I here so much sloppy and indistinct slide guitar? How do you avoid this?"

Because like with anything else, skill shows. Being able to mute, with both hands, and only have what you want to be heard be heard takes years of dedication. But to the uninformed bad slide playing may sound bluesy. And for some it's such a novelty that the playing of someone who dabbles once or twice a week sounds "cool".

I am reminded of the chef Mario Batali who more than once has said "Dude... That's not rustic... That's lazy..." When commenting on the chopping of a prep cook.