Most of the negativity stems from id's new Doom not feeling much like id's old Doom. Common complaints include the weapons not feeling punchy enough, movement speed not feeling fast enough, and an objection to the inclusion of things like loadouts and progression. I've seen Doom's rocket launcher picked apart, too. It doesn't do enough damage, players say.
The "most useful" Steam user review, posted by someone who's played the new Doom for over four hours, says it feels like "Master Chief with a squirtgun on a shopping cart".
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It looks like the Steam user reviews were driven largely by player expectations, then. Those expecting an old-school arena shooter may have felt misled. Conversely, those going in fresh may have felt pleasantly surprised.