Ever wonder why that Guinness you're drinking has such a smooth creamy texture? It's carbonated with nitrogen as opposed to Carbon Dioxide, which carbonates most beers and soda pop. Nitrogen bubbles are smaller, creating a completely different texture and much prettier foam cascade when the beer is poured.
Nitrogen carbonation changes the flavor, too. More aroma is released, which does wonders for ale styles (and some lagers. Boddington's comes to mind). Some bars have dedicated nitrogen lines through which they rotate a series of beers. If you find one of these in your neighborhood, treasure it and become a regular. A whole new world of beer enjoyment awaits.