1) Digital output (on player, no sub-packs or mini-docks!)
2) True line-out (on player)
3) Decent headamp
4) Warm tonal balance (most portables and IEMs are bright)
5) Multiple lossless codecs supported: wave, FLAC, ALAC, also .ogg support for lossy files
6) 60+ GB, or as much HD as is possible
7) 30+ hours battery life
8) Organization by folder
9) No need for any extra software (i.e. drag and drop USB mass-storage compatible)
10) Small enough to fit in pocket with a pair of IEM's
11) Nice long warranty
12) An EQ that can be used subtractively and doesn't distort or degrade SQ
13) Gapless
14) Removable internal battery (doesn't have to be a stock AA or something similar, just a removable battery like on my old Zen Micro. This way, I can take multiples along with me and extend battery life)
What I really don't care about:
Color screens, photo and movie support, those ultra-intuitive controls, style and looks, FM tuners, recording and line-in, BBE and similar sonic gimmicks, useless peripherals (remotes, etc).
Basically, I want a really good, barebones player that's designed from the ground up to be used with high-end portable and stationary stuff. I also want to have the ability to sample sound at any point in the signal path (true digital and analog outs).
I'm guessing that this will never happen, and these conditions will never be met. I don't think that with the current technology, a player can have a high-quality DAC and headamp, provide 30+ hours of battery life, and still fit in a pocket. Unless there's some new tech on the horizon that I'm not aware of. There isn't a large enough market to invest in the kind of research that one would need to produce this kind of player, and the most we can hope for is a trickle-down effect from other industries that lets us assmble existing pieces into what we need.
But, real digital and analog line-outs are nice. I wish more players had them. They're not all that hard to implement (I think?). Removable batteries are great too, I wish more players had them.