The rep system has been turned off, because it is too easily gamed. (There's no guarantee that the votes represent true quality.) There's nothing to stop you agreeing, or disagreeing, as part of the discussion—but it's a better discussion if you say why. (If find that other forums, where posts move up and down according to how many votes they get, horribly confusing. You can't follow the argument at all.)
The tagging and mentioning tools are handy for alerting other members about a discussion. For example, say you were discussing CSS and wanted our resident CSS guru, Paul O'B, to be aware of the conversation. If you give him a mention, he'll be notified and come see what's cooking. There are three ways to do a mention:
1) type the username with an @ in front and a ; following
2) manually type [mention][/mention] tags around the username
3) click the Go Advanced tab when posting, highlight the username and click the Mention button at the end of the editor tools ... which looks like a blue eye or something.