Reno's appearance is interesting because it shifts so from the actual game to Advent Children and the rest of the compilation. In the Final Fantasy VII official art, he's a great deal more sleazy. He doesn't smile in his picture, it's more of a leer, and his cheeks are sunken in making him look tired, hung over, or possibly both. His clothes are unkempt, but it could be due more to general slobbishness and irrespectability than to a cheeky disregard for rules.
Reno's posture is worth noting, because to me it's the most striking thing about him, in a game with graphics as early as Final Fantasy VII - Reno carries himself differently than every other character in the game. He doesn't stand up straight at all- rather he has a kind of arrogant swagger to him. He looks as if you're wasting his valuable time. It's cocky and laid back both at once, and it's why I immediately took interest in the character.
In Advent Children, Reno isn't leering at all, in fact, all of the compilation promos feature images of him with his tongue sticking out. His face, which looked sunken before, is more fleshed out. He's back, he's wearing black-- in Advent Children and the rest of the compilation, the Turks wear black, not their slightly more tailored blue jackets. Anyway, his two scars (or, more likely, paint marks/tatoos) are more clearly visable in all their uniformity. But his shirt is still unbuttoned, his hair is still wild, and he's still full of morally ambiguous charm.
As for the scars/markings/tattoos/whathaveyou, I have no idea what they are or what their purpose is. Before Advent Children was announced, I had always assumed them to be scars, as that's what the one peice of official artwork led me to believe, but they are redder and more uniform than most scars have any right to be. It's not clear what importance they have, but Reno has had them since Before Crisis and into Advent Children, so he clearly considers them important. Or else he did that drunk one night and can't get rid of the consequences. Who knows? They're just part of what makes him distinct. He doesn't strike me as a heavy symbolism kind of guy.
Like all the Turks, he wears the standard issue uniform. Unlike the rest of them, he leaves his shirt unbuttoned and leaves the tie for the Avril Lavigne wannabes. It may be a uniform, but he's going to take the standard issue and take it up to standard. Oh yeah. He's just classy like that.