First off, he's massive. The scale alone is impressive. And when you consider that he's literally one giant slab of marble, its even more so. Michelangelo was a beast, y'all! Secondly, the proportions are so wacky in person, that it's nothing like seeing it in a textbook.
Even with four years of art history under my belt, it was still nice to go there with an audio guide -- specifically Rick Steves' Guide to Florence's Accademia. His unique take on the narrative was something I hadn't heard before. Give yourself a treat, and listen to it yourself! It's better if you're actually there, but it's still interesting even if you're not.
The Accademia is actually super tiny and besides the David, the main event is the so-called Prisoner series, which are basically figures midway through the carving process, trapped in the marble. It's fascinating to be able to see Michelangelo's process literally captured in stone.
Here's the rest of the goods (including a few more of my beloved David):