I played the beta for quite a bit. As someone who completed the game with 50% completion, and this being my first Metal Gear Solid game, I loved the story, but the beta is something else. Where Metal Gear Solid is Tactical Espionage Action, Metal Gear Online focuses on Tactical Team Action, with two teams: Solid and Liquid.
Each team has 1 unique character for now. Team Solid has the protagonist of Phantom Pain, Punished "Venom" Snake, who excels in CQC and has a rocket arm, as well as having the amazing Kiefer Sutherland as his voice actor. Liquid's unique character is none other than Revolver "Revolver "Shalashaska" Ocelot" Ocelot, and of course, his revolver.
Other than the two unique characters, the rest of the players are custom made avatars, each with unique loadouts, and a store to purchase cosmetics, such as eyepatches, Kojima's glasses and a golden swimsuit(?).
And then there's the music. Oh boy. There are two types of music in MGO, sets and songs. Songs loop for the duration of the game, while sets work differently, acting like most songs that can be replaced. Here's a list: