Finding out what happened to the Protheans
Mass Effect 2
The reconnaissance mission aboard the Collectors ship in Mass Effect 2 is a pain in the ass, but it's also crucial to the plot. Its spoils allow the gang to traverse the Omega 4 relay—and it leads to the discovery of the Protheans' true fate.
They've been mostly extinct for 50,000 years, after all, so it was quite a mystery. But it turns out the Reapers wiped up the survivors once they were done and turned them into cyborg slaves. Naturally, Mordin's got even more info later on. Mystery solved! Next we'd like to find out what happened to the Dwarves in The Elder Scrolls. Thanks!