system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars ROTJ] Why did everyone get all somber/scared during the mission briefing when Mon Mothma said the Emperor was personally overseeing the final construction of the DSII?

Because he's old and creepy looking?

Because he's reputed to have mystical powers?

Because he's the big boss and this is a chance to take him out?
gpt
Because they knew without a doubt that his presence there would mean that their job would be much more difficult.  Everyone there fully understood that this was an all-out, do-or-die effort.  
Think about how starship battles work.  Just about every ship is capable of escaping into hyperspace with only a few moments notice.  Bigger ships might take longer to line up to the route, but they can generally tank the beating they might be taking.  Full scale battles with heavy losses on both sides are very rare.  The rebels jump in, smash up a few targets before things get too hairy, and jump out again as the empire brings in reinforcements.  That's how 99% of the battles of the galactic civil war were fought, and the rebel fleet was *good* at it.  They had fast, small, nimble ships, excellent plots, and usually had surprise on their side.  
The fight they were heading in to wasn't going to be like that.  They knew that they were going to have to stay and go toe to toe with whatever was waiting for them at Endor.  They were already planning to accept heavy losses, since even if their efforts to distract the imperial fleet were successful, there would be a force in orbit that would be stronger than just about anything they had encountered before.  And that was *without* the emperor.  *With* his presence, it probably felt like they were planning an attack on a military parade.