1) Are you currently looking for or using a music server?
No, I am not, as there are a few other things in my listening system I want to get taken care of. I see this as a nice addition to a complete system, so yes, I would definitely love to have one. Right now I'm using an awkward combination of the USB-in of my receiver with an external hard drive and an mp3 player, and the HDMI out of my laptop complete with video handshake headaches, and my PS3 at times. Ghetto, to say the least, but it'll suffice until i've got a subwoofer, an external amp, some bass traps, etc. "Get the sound right, then get the sound easy."
2) When purchasing a music server, which are the three most important considerations to you and why?
Addon plugins. As someone who uses foobar2000 for his music listening, i've been subjected to some amazing plugins. I don't know if I could live without some of that stuff. In fact my dream music server would basically just be based on the foobar2000 platform altogether.
Easy to use GUI/screen - As a plasma owner i don't want to use my TV screen or even really deal with HDMI handshake and I definitely don't want to spend 20 minutes trying to find the song I want because it just happened to be tagged poorly. I want lots of control over organization basically. Likewise, I expect fast processing and no lag.
Use as a purely digital LPCM transport- not that i necessarily mind a high end DAC, but I feel digital is the way to go. Especially since a receiver or processor may want to do its own processing for whatever reason (IE room EQ), so I don't want to cascade a DAC with an ADC and another DAC.
Those three are the most important, but if I had to add one more, it would be that the music server can cooperate with my portable music player so i can move files to and from.
3) Are their any deal breaker features in music servers (meaning you wouldn't buy one with that feature)? What are they?
*Are there, you mean.
Anyways I would definitely never buy a music server that has poor firmware support, infrequent firmware updates, or worse of all, un-upgradable firmware. The last thing I want is to be stuck with an electronic that has one nagging issue which could easily be fixed with a firmware update that never will be. Feedback and a focus on improving performance are something which draws me towards some electronics companies over others.
Inability to play FLAC / ALAC / APE would probably be the other one. It's all about the lossless.