Clickable links
I agree with the your pros, and generally when someone has an idea the majority of the time their intention is to usually to make things easier for others, or almost completely effortless.
If this game didn't have years of involvement with younger audiences, then I think it would be a really beneficial feature for role-play/group advertisers and people who just don't want the hassle of manually typing links into their browser. Buuuut since this isn't the case I don't think it would be safe to have considering how much more popular link posting would get with how easier it would be to do, and the type of links and sites out there, and how naive/gullible younger audiences can be. But, you have partially said/understood this already so I won't continue. I just wanted to add onto it.
If the circumstances were different, there are some ideas/ways around the cons you have listed to generally improve the negatives of the idea you have already posted. Developments that would make things safer. Not 100% "in the clear" safer, but just safer in general. From the top of my head, the ways public safety could be improved if this were to be implemented would be:
1. Only allow clickable/active urls/links via private chats such as whisper, group, and party.
2. Advise users in the rules to not click on any links from users they do not know, links provided randomly, or links from sites they do not know etc.
3. Have the chat automatically block links to sites that are blocked by the game (if possible, like how the forum blocks tinypic links).
4. Have a system where you declare your age, and users over a certain age are able to click on links. Under-aged users would see a blocked message where the link would be pasted. If users lie about their age then they are only responsible for their own actions. But I feel this would be a fairly complex thing to do.
One of your cons say "If the link is too long so it goes over the characterlimit, it may not work, and also flood the chat", but I don't think this would be a problem as there is a character limit so long links won't exceed over the character limit even if it were pasted. Unless you were thinking of something else that I'm getting lost about? xD
But yeah, that's what I have to say about this idea in particular, and what I have to offer in terms of thinking of every possible improvement before declining the idea at first sight as people sometimes have the tendency to do.
Copy and paste text instead of typing it in manually
Along with your pros, this would help a lot with communication with non-English speakers. I also agree with the spam but if there was some sort of cool down or something that'd be good. Plus we have moderators. xD
Chat commands
Instead of /rules or the /link commands, I think an extra button for the rules on the in-game login screen would be more accessible and in-your face. But I do agree that there could be some really good use for in-game commands, and Deucalicorn and others shared some really good ones!
All language letters/symbols
Goodness this would be great. And again, like idea #2, this would help a lot with communication with non-English speakers IF users could highlight text in the chat and use copy methods to paste into a translator, and vise versa. This idea would co-exist really well with idea #2.
Chat color codes
What xSpirit said.
