There are few but some client only mods such as theme packs. There are some but few that are client optional. Basically if they aren't installed on the client the person doesn't have the ability to use the mod. Most must be on both. The long term plan is to have some sort of mod repo. Then when a user selects a server it'll automatically download the required mods. For now it just tells you what mods you are missing to join that server.