I played on a server with a friend the other day, and there were a load of great mods.
A good one was an RPG mod where the more you mined fished swung your sword etc, the better you get, and you get a chance of procs, like you can be mining coal , and you have a chance to find some diamond or iron too.
Also a shop mod , where you have a bank ballance, and you can sell stuff like red stone , and buy wood or gold or whatever you can afford - sortof makes use for those materials you dont need.
There was also a really anoying mod (or i thought) that was too hand holding, that stopped creepers destroying anything and just damaging instead.
Generally, any mod that expands the game is great in my opinion, but some stuff can be restrictive, ruining some part of the game for someone.
a mod that detects what blocks were placed where or who destroyed what would solve most griefing problems, - along with a white list these are the only restrictive elements i would like to see.
not my server i know , just me 2p