Simple as it comes, did you like the ODST or Reach gamemode, firefight?
And if you don't get why I'm asking, take a minute to reflect on the fact that I posted on the Destiny forum.
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I really liked Firefight, especially the ODST variant that had limited supplies of lives and ammo. I do think a key difference between Firefight and POE is that Firefight had the semblance of a story-motivation to it. It was areas from the campaign, and you're fighting waves of enemies that had a purpose in that arena. Not sure we'll get much of that in Prison of Elders.