Wait, the Mass Effect and Halo plots are nothing alike. One is about humanity fighting a group of space-catholic aliens who hate humans because of their religion and try to exterminate them. Oh, and on the way a bunch of headcrab zombie aliens are introduced. Its the story about one super-soldier who somehow changes the entire course of an interstellar war.the main plot is pretty unoriginal. I'd even say that it's a weaker version of the Halo plot.
Mass Effect is about a soldier and his/her crew coming across the knowledge that the people of the galaxy have been manipulated and tricked by a super-advanced race of what are essentially mechanical lovecraftian horrors, who intend to kill and harvest them all. Of course, everyone thinks Shepard is crazy, and these warnings fall on deaf ears. You also have the whole "humans are the new kids on the block and are trying to earn the older species' respect as they interact with them more".
The stories are completely different in tone and themes dealth with.
...Okay, this is just stupid. You can't really compare a bunch of mile long cosmic horrors to a swarm of headcrab zombies.The reapers are the flood.
One is a heavily lovecraft inspired race of massive intelligent space-faring organisms who manipulate mortals, drive them crazy and turn them into mind-slaves before coming around and harvesting every intelligent species in the galaxy to make more of themselves. They are interesting, they are intelligent, and they have distinct (albeit still somewhat mysterious) motivations.
The other is *gasp* an alien bio weapon/plague thing that got out of hand and turned on its creators. Like that hasn't been done before (and better) in starcraft, the alien series, and warhammer40k decades before Halo was a thought in anyone's mind.
The tyranid and zerg were the flood before the flood were the flood. Hell, headcrabs from halflife were the flood before the flood were the flood. Don't accuse a series of being unoriginal by comparing it to another series that has heavily borrowed elements from everything from 40k to halflife.
To be fair, no sci-fantasy setting is completely original. The whole point of sci-fantasy is to take fantasy elements and reinterpret them within a futuristic setting. Execution trumps originality. Take the original Star Wars trilogy. The plot was really just the classic hero's journey reinterpreted in a futuristic setting. It is FULL of tropes and cliches, but they were executed well, and had enough on a new spin on them to avoid feeling completely rehashed.