Perhaps it's past the point of spoilers by this point, so now we can openly discuss the events of Avengers: Infinity War, in particular the cliffhanger conclusion and the "butcher's bill" attached to its final battle. It certainly became a meme online with the words "I don't feel so good".
So here's a list of prominent casualties suffered in the Marvel Cinematic Universe at the end of Infinity War. It's going to be an interesting question of whether the losses will be undone by Avengers 4 in 2019. Not included here are losses on the side of the culprit, Thanos.