James Lileks (a humorist): How We Learned to Stop Worrying and Love The Light Rail
"Now, to repeat the point often made here, with tiresome regularity: what’s the matter with better buses? Seriously: If we’d kept the old streetcar system, maintained all the cars and updated them with A/C and comfy seats, our system would be renowned nationwide as a model for all. If we’d converted the trolleys to diesel, taken down the unsightly wires but kept the old cars looking exactly as they did in their glory days, it would still be revered as a model of inter-urban mass transit, because it ran on rails. Is that all it takes? Rails? Do they impart some particular magic?"
"Is that all it takes? Rails?"
Nah, not the rails -- it's the dedicated right-of-way that allows people to zip by congested traffic. Rails just tend to be associated with said ROW.
Buses with exclusive guideway aren't necessarily the answer, either. If you you were able to convince all of the riders of Houston' LRT to cram onto BRT, for example, you'd have to run ridiculously short headways which would royally screw up cross traffic. I don't think that'd make ANYBODY happy.