Guinness is a personal favourite of mine and is well known for having an amazing degree of brand loyalty from its customers, partly as a result of the interesting stories told about it.
Guinness, like many beers, does have a reasonably high calorific content. However, it is not 'a meal in a glass', 'liquid bread' or anything similar. Draught Guinness contains 43 calories per 100ml or 244 calories per UK pint. For comparison, orange juice contains about 220 calories per pint and milk has about 260 calories per pint.
A pint of Guinness will have the descending bubbles but so do most dark beers. It's nothing specifically related to Guinness.
The stuff marketed as Draught Guinness (cans and apparently bottles but I've never seen them) is different from the Original (cans and bottles as well) in that the Draught has a widget which contains a pressurised nitrogen/carbon dioxide mix, the same as the gas mix pubs use. When the container is opened the gas mix is released, sending the gas through the beer. In contrast, the Original contains only carbon dioxide to give the fizz. This is the reason why Original Stout has a much more bitter taste than Draught as some carbon dioxide dissolves to give carbonic acid. The Draught tastes creamier because there is less carbon dioxide and so less acid (nitrogen doesn't react with anything in beer)
Interestingly enough, Guinness is not suitable for strict vegetarians because the fining agent (for getting all the crap out of the brew mix before they bottle it) they use is isinglass, a form of gelatin obtained from fish.
The rumour that Guinness turns your poo black is true, after heavy consumption (usually the morning after Saint Patrick's Day) Guinness poo is a common occurence. Why this is is not known for sure, it is usually hypothesised to be staining by the colour from the roasted barley or reaction between iron and bile products in the digestive system. I haven't seen any proof for either.
On a related note, Guinness is not black. Next time you have a pint hold it up to the light and the deep ruby red colour will become obvious.
Finally, it possible to create multiple layer drinks, usually called a Black and Tan or Half and Half. This is usually done with an ale on the bottom and Guinness on top, poured over a spoon to avoid disturbing the ale.