Ebarnacle, it was Aragorn, not Gimli, who said "Let's go hunt some orc." It might have been better coming from Gimli, actually.

Grab, I think the half-assed look of the assault on the Black Gate might have to do with the way the film was cut. In the book, when Aragorn rides up and bangs on the gate, one of Sauron's lieutenants emerges and negotiates with them. He shows them Frodo's mithril shirt, Sam's sword, and another artifact (can't remember which...his elven cloak?) and claims that he has Frodo in captivity. He threatens to torture Frodo to death if Gondor does not capitulate. In the meantime, of course, Frodo has escaped leaving those artifacts behind, so the lieutenant is bluffing.

In the movie, they went to some trouble to show Frodo's stuff, including the mithril shirt, being taken away by Orcs, which is mostly significant as a set-up to this absent scene. So I suspect that this scene was filmed and then cut, and will be restored for the extended DVD. What PJ was left with, then, was footage of an approach to the gates for a parley, not an attack. In this version of the film, the parley never occurs and the battle begins immediately.

As for the rest of the army, including the elves and the rohirrim, they might in fact be just over the next rise waiting for the proper moment, which does not come because Frodo destroys the ring in time.