The timbre of the upper mids and lower highs sounds off compared to other offerings from sennheiser, beyerdynamic and denon. It shares the same characteristic as the AD700, but I've heard people say the AD900 fixes that issue. I don't know the exacts of it but I guess it's how even/uneven its frequency response is in that area of sound and how it messes up the sound of instruments' natural overtones.
The punch of the headphone seems lacking compared to the DT990 and D2000. Even though it has more bass quantity than both it has less physical impact to it, but now I'm nitpicking by comparing it to 300 dollar cans.
Soundstage is probably something 9/10 head-fiers would label as one of their top sound quality indicators, but everybody has a different preference in priorities. I know it's why I don't like the M50.