This never occurred to me until right now. I've always considered Lando an interesting character but a secondary hero in the saga, relatively minor. But just now I was posting in another thread about why Admiral Ackbar ain't the end-all be-all when I realized that, were it not for Lando, the entire Rebel Fleet would have run from the fight over Endor under Ackbar's "leadership" not once but TWICE!!! Not only does Lando figure out the shield is up before it's too late, but it's Lando's decision to stay in the fight even with the shield not yet disabled, and then it's his call to take the fight directly to the enemy fleet when faced with the giant scarylaser, all which allows the Rebels to execute their final attack on the Death Star, not to mention crippling the Imperial Fleet in the process. Ackbar wants to cut & run the moment he realizes the shield hasn't been downed, and then he doesn't seem to know what to do as the Death Star's laser is picking off the capital ships one by one, he just figures they should let them hang out with their stuff in the breeze with no cover at all. (As a side note, the Emperor would have seemed even more badass if he had let the Death Star superlaser shoot a Rebel capital ship that was super close to a Star Destroyer, thus destroying them both.)
Whatever Leia and Han and even Luke did at the moon of Endor, it all would have been for naught if Lando Calrissian hadn't stepped up and taken charge of an otherwise poorly-led Rebel fleet!