For the most part, the better-sounding death mixes don't sound uber-different when they're leaving. Usually 'bigger' (chest up, shoulders back, neck straight and what not), I'd say typically a little less fatiguing - Much of the death stuff that comes in here isn't super clean with gobs of headroom -
Suffocation's last disc was an exception - I can ask about putting A/B's up, but don't count on getting the okay from Nuclear Blast on that one... That recording was very clean, lots of headroom (as almost anything I've ever worked on that Zack Ohren had a hand in typically is).
Typical issues I see with ("rookie" for lack of a better term) death bands is the guitar tone... They set their amps up to sound like "this guy's" tone - But it's not "this guy's" tone - It's "this guy's tone through a stereo" (which isn't what that amp sounded like before). And (of course) - tracking way too hot. Easiest way to screw everything up from the very first step.
Long story short - Before or after mastering - it either sounds like a good recording or it doesn't. The mastering guy doesn't 'decide' that - He might have to 'deal' with it, he might have to tweak it, he might have to control the damage inherent to the customary application of 'war volume' - But the mixes should sound like great mixes before the mastering guy has to do anything to it in the first place. They might be *a little* more 'dull' or a little less 'polished' or what not - But they should still be solid (figuratively speaking, of course).
