There are a few variables that keep cropping up, and they seem to be an indicator to me of what makes one player better than another. They include:
Signal/Noise (S/N) ratio. The Cowon J3 has a 95dB rating. I can't find that value for most other players, but rarely is their number that high.
Output power. You want the player to move your headphones. Cowon J3: 29mW/channel. Again, this is a hard value to find for many players. The Samsung P3 is rated for 20mW/CH. Mine P3 doesn't drive my large over-ear headphones as well as my J3 does.
Battery life. You don't want to find the battery dead when you're in the mood for tunes. Cowon J3: 64 hours. iPod Touch: 30 hours. Samsung P3:also 30 hours.
Loss-free codec support. My J3 will play FLAC, WAV, and WMA lossless. The Touch does Apple Lossless and WAV. Windows media player will rip to WMA lossless.
Construction. You don't want your player failing. Samsung P2s had a high frequency of headphone jack failures. I've had two Touch seize up (White Screen-Of-Death) and require replacements. My Cowons keep on ticking, despite naive critiques associating their light weight with low quality.
Control. Nothing is worse than a player that makes you jump through hoops to change songs or simply pause the current one. Most will shut off the screen to save battery power. Once that happens with a Touch you have to at least double tap Home then tap the Pause/play icon. With the J3 you simply press the external pause/play button. Two extra aggravating steps for the Touch.
Settings. A Touch forces you to exit the music app, open settings, then swipe until you've found audio settings. Change what you want, then exit Settings and re-enter the music app. Almost every other player has audio settings accessible directly from their music app.
These are (a few of) the reasons I don't care for Apple products as music providers.