What is the difference between a Mac mini and a Mac mini server?
Comparison of Mac Mini and Mac Mini Server (as of mid 2011)

Processor:


Server: Quad core i7
Mac Mini: Dual core i5 


The base model of the Mac Mini does not support AES-NI which is recommended when using FileVault 2.



Graphics:


Server edition and Mac Mini base model: integrated HD Graphics 3000
High end Mac Mini: dedicated graphics, AMD Radeon HD 6630M with GDDR5


Note: This is very recommended for Lion on large external displays. The integrated HD Graphics 3000 do not perform well in Lion as of 10.7.3 - even on my 15'' MBP.



Hard drive:


Server: 2 x 500GB at 7400rpm 
Mac Mini: 1 x 500GB at 5400rpm - and thus quieter


Optical Drive:


Server: none
Mac Mini: none


Weight:


The server edition weights 0.3 pounds more.


Video, Audio, Network, Dimensions:


same