This topic has been brought up every now and then in the past, but I'd like to do it once again. Some of the allowed mods give players an definititve advantage that is unfair in my opinion. Players should be able to use an unmodded client to play the game.
Health Tags
Mods that show your opponents health are a rather big advantage. You can easily disable them by sending wrong health values to the client. Another option would be to add server side health tags for everyone but I think that's a bad idea and will negatively affect the games. Doing nothing is even worse though.
Armor HUD
I would support banning armor HUD, but since it's basically undetectable that wouldn't be feasible. In games that include your armor breaking, it's a huge advantage to know so beforehand, allowing you to back off or eat a golden apple. To make it at least a little less unfair, you should add a sound that is played when your armor breaks. This should be in vanilla minecraft anyway, but I think it would be pretty easy to just implement that feature on the server instead of waiting for it to be fixed.
Fullbright
It's possible to change the brightness in the config file even without using mods, but that's pretty inconvenient, especially if you play on other servers that don't allow that so you have to change it back and forth. To give players equal chances on night maps and UHC's you could add a command that toggles the nightvision effect. This may sound like it will make the game too easy, but you have to keep in mind that some players have this advantage already.
Health Tags
Mods that show your opponents health are a rather big advantage. You can easily disable them by sending wrong health values to the client. Another option would be to add server side health tags for everyone but I think that's a bad idea and will negatively affect the games. Doing nothing is even worse though.
Armor HUD
I would support banning armor HUD, but since it's basically undetectable that wouldn't be feasible. In games that include your armor breaking, it's a huge advantage to know so beforehand, allowing you to back off or eat a golden apple. To make it at least a little less unfair, you should add a sound that is played when your armor breaks. This should be in vanilla minecraft anyway, but I think it would be pretty easy to just implement that feature on the server instead of waiting for it to be fixed.
Fullbright
It's possible to change the brightness in the config file even without using mods, but that's pretty inconvenient, especially if you play on other servers that don't allow that so you have to change it back and forth. To give players equal chances on night maps and UHC's you could add a command that toggles the nightvision effect. This may sound like it will make the game too easy, but you have to keep in mind that some players have this advantage already.