First of all, lets start off with a little debunking about what Guinness is and is not.
It is not motoroil. It is not heavy or thick. It does not have a harsh or overly bitter flavor. You're thinking of IPAs.
Guinness is light. It is creamy and smooth with only the slightest hint of carbonation. It is malty with undertones of coffee and chocolate. It is rich tasting, sure, but that should not be confused with being heavy. If you have tried porters or other stouts - especially flavored ones - and been put off by syrupy finish, don't be. Guinness is a stout unto itself. Comparisons cannot be made.
While the brew is certainly a bit dense due to the lack of excessive (read: harsh) carbonation, Guinness is in fact so light, that when mixed with other brews, it floats on top.