For a roleplaying game, D&D provides scant material for defining your character's personality, other than his "alignment". With other RPGs, you can pick from lists of virtues, flaws, quirks, mental disadvantages, background options, etc., to flesh out your character, making him truly 3-dimensional and roleplay-able. But D&D doesn't have any of that built into its core rulebooks.
What D&D does have is a plethora of rules for combat. In fact, rules for combat take up about 2/3 or more of the Player's Handbook. (Consider: The bulk of spell descriptions, even, is for how those spells work in combat situations.)
D&D is first and foremost (though, of course, not entirely) about dungeon crawling. After all, "dungeons" is the first part of the game's name. Sure, you can use the D&D system to run a true "roleplaying" campaign. But, IMHO, roleplaying systems such as the Storyteller System (Vampire, Mage, Changling, etc.), Ars Magica, and Pendragon are better from the get-go, for that sort of thing.