For most people, volume=quality. If it can get loud without distorting, and "really thumps" they'll be happy. Clear that bar, and it's all about the non-audible pieces of the experience.
2. Don't otherwise hurt the user (there were cases of earbuds and over-the-ear headphones where the maximum volume would damage the wearer's eardrums, for example).
3. Easily and reliably connect them to the desired device.
4. High quality enough that the product lasts years.
5. Trusted by users that 1-4 apply.
Most audio brands are obscure for the average user. If I ask someone random on the street about Sennheiser they won't know about the brand.
