Apple released a new Mac mini model in October, and lot of important details have changed.
There is no longer a Mac mini “Server” option. If you want to run the Server app on a Mac mini, it must be purchased through the Mac App Store for $19.99.
The new Mac mini only supports one internal drive, so the days of purchasing a CTO from Apple with dual internal drives are over.
Firewire has been dropped, and replaced with a second Thunderbolt port, requiring a Thunderbolt adapter if you want to connect Firewire devices. On the positive side, Thunderbolt has been updated from Thunderbolt 1 to Thunderbolt 2.
Quad-core processors have been dropped. All new Mac minis have dual-core processors, and ship with Yosemite installed. Mavericks is not supported.
The memory is no longer user upgradable, and instead soldered to the motherboard, so you are stuck with whatever RAM option you choose when purchasing the Mac mini. Furthermore, the base model only comes with 4GB of RAM, which in my opinion is not enough for anything but extremely basic Yosemite apps (and Adobe CS/CC will completely choke with just 4GB of RAM). When purchasing a new Mac mini, I would stress that buying the 8GB or 16GB RAM configuration is an absolute must.