Some of the qualities I've found first are:
1. The sound. The most important factor in a headphone. No pink paint gloss, sponsorships, or any of that non-sense. It is a very clean sounding headphone that produces bass and all other frequencies very well. No sibilance at high levels of play. Running this out of a Xone 92, the sound that comes out is pure sultry warmth.
2. The fit. They fit snug without cutting off circulation. I can be sensitive to headband pressure, and these have just the right amount for a dj. I am using the velour pads as they are way more comfortable as well as more breathable. They have some memory foam qualities to them.
3. The build. They are solid. Just like the HD25, these are tanks. They are heavier, but for what it is worth, it is purely for function. The ear cups can swivel if you need, and they click with assertion. Build and stability is a very important quality for a dj. DJ's can sometimes be careless with headphones after mixing songs together and may not always place the headband around the neck.
4. The accessories. The cables, the extra pads, the case, etc...alot of companies are throwing additional accessories as well, but it's quality material. It's always nice to have the little things included.
5. The warranty and brand. 2 year warranty. Built by Sennheiser, known for making high-quality headphones and having excellent customer service. Everything Sennheiser I've owned has lived up to it's standards.
For me the only drawbacks are: The length of the straight cable. Very long and almost impractical. No DJ would ever use this. I've also experienced occasional clipping at maxed out levels with my xone92.