Classically-Trained Extra: Not in the usual sense. General Garza's two bodyguards, who look mind-boggingly similar to one another? Twin MMA fighters Antonio Rodrigo Nogueira and Antonio Rogerio Nogueira, who currently compete in the UFC alongside Randy Couture. Sadly, they're not seen in any hand-to-hand combat.
Doing It for the Art: Jean-Claude Van Damme refused to be in the first film without seeing a script first (which is why he wasn't in it), and even his role in the sequel was largely based on what he wanted to do with the character. In particular, the final fight between him and Stallone was originally going to be stunt-heavy involving a helicopter, but he insisted on a straight-up fist-fight because he knew his character needed to die in a very satisfactory way. Vilain even breaks the fourth wall during the fight when Ross struggles to get back up to his feet. "Is that all? The people want more for their money!"
B Movie action fans (and there were probably a few in the audience) will recognize Gary Daniels as The Brit. Some may even recognize him as Kenshiro from the live action adaptation of Fist of the North Star.
Playing Against Type: Many Dexter fans were surprised to see Angel Batista, one of the most consistently friendly, even-tempered, decent characters from that show, as a military dictator exploiting his island to fuel the drug trade.
Christmas acquiring a ruby from Burma. Alfred relates a similar tale, and closer to home Burma was the setting of the most recent Rambo film, starring...you know.
The drilling vehicle that Trench (Arnold Schwarzenegger) uses to bust the Expendables out of the collapsed mine bears a STRIKING resemblence to the drilling vehicle that Quade (also played by Arnold Schwarzenegger) shoves a drill into in Paul Verhoeven's adaptation of Total Recall (1990). And of course, the homage to his actor's most beloved Catch Phrase as soon as he appears:
Throw It In: Bruce Willis ad-libbed his jab at both Arnold and Sylvester about going down on each other; Stallone was initially disgusted, as revealed in commentary, but the line set the scene so well that it was kept.
Also, in the first film, the seaplane was supposed to be an unarmed escape vehicle, but in between filming, Stallone noticed one of the crew climbing out from the nose hatch, learned that it was accessed from inside the plane, and was immediately inspired to turn the seaplane into a gunship.
What Could Have Been: As mentioned in the description Seagal and Van Damme were offered roles but they turned them down due to problems with the producer and a dislike of his character, respectively. Also, Terry Crews' role was written with Wesley Snipes in mind. The role was almost filled by Forest Whitaker, but had to drop out due to scheduling conflicts. Kurt Russell and 50 Cent were offered parts but declined, and there are rumors that Chuck Norris was approached as well. IMDb claims Danny Trejo, Denis Leary and Alan "Wash" Tudyk were approached for certain roles that were written out when they had scheduling conflicts. Stallone wanted all of them if he could so we aren't sure if he wanted the universe to explode.