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I can't really think of any single mammalian behavior that universally differs in kind from the rest of vertebrate behavior, only in degree. For example, while mammals in general may communicate to a greater degree of complexity than do amphibians/reptiles/birds in general, amphibians/reptiles/birds do communicate.

One aspect of mammalian behavior that seems to be isolated to mammals and birds is parental investment behavior... while some reptiles will guard nests or even give birth to live juveniles,

(http://encarta.msn.com/media_461543817/Snake_Giving_Birth.html)

there isn't really an infancy period during which these animals are nurtured or cared for by parents. This infancy period (and the resulting parental investment) is seen in mammals, and birds. (Even precocial birds like quail or chickens are cared for by a parent) If the jury is still out on the question of bird-neocortex existence, then perhaps this aspect of behavior could be attributed to that portion of the brain.

On the idea of parental investment/maternal instinct, I'd like to leave you with this video. (If this link doesn't work, try googling "Hippo Saves Impala" and checking the video search results.)

(http://www.youtube.com/watch?v=WngCnl6OcOQ)