I'd like to start this topic by mentioning that I do not know if there has been a topic about this already, and if anyone wants to add their two cents then please feel free to do so :) (I'm sure you will anyway)
Too many times I've had my train of thought derailed by some silly person who thinks it's perfectly sociable to randomly add someone in-game because they like what they're wearing. I think that the least that should be implemented is a filter for the mail system. This should be fairly easy to code because it's a simple User Interface modification.
Also, I would like the ability to block trade requests. I've lost count how many times I've come out of a long dungeon run only to be confronted by some greedy newbie that wants a handout because he's new, or because he's just 400 crowns short of a new weapon in a series of new weapons.
I think that's pretty much all that I wanted to mention at the moment. if I think of anything else, I'll add edits above this paragraph. Again, I don't know if there is another topic like this already, but obviously it hasnt been done yet, so I am making my own topic right here.
Two small checkboxes in the UI somewhere could easily alleviate the random friend request / random trade problem.
It does get mildly annoying to occasionally have someone meander up and request a trade just to ask for x amount of crowns or crystal energy, but the fact that there's a conveniently-located no button kind of leaves this suggestion with something to be desired. Perhaps a filter for who is allowed to initiate a trade with you (public / friends / guildies / nobody / combinations of these) would be more appropriate?
As for blocking friend requests - I guess if you really wanted to go out on a limb, you could take the facebook-esque (and its many clones) approach (friends-of-friends only, etc) with filtering incoming requests, but most people would find such a feature to be useless or redundant, again, because of the already existing relatively easy series of clicks to enter your inbox, open a request, and simply hit delete. The fact that multiple / spammy requests already get filtered out/ignored is a testament to ThreeRings having addressed this issue at least once in the development process, so expanding on it may or may not be a good idea. The currently existing system works, and the annoying popup boxes for trades only happen in Haven, where it is highly unlikely that you will be eaten by a Grue just for standing still while you attend to your inbox, if only because of the sheer number of other people who would probably be eaten first for breathing loudly.
My personal opinion is that a checkbox/dropdown for trade filtering would be great, but I don't mind having strangers on my friends list (at least in Spiral Knights, where I don't feel threatened at all by the concept), and I'm fine with the already existing decline option for friend requests being located in the inbox, where it is out of my way while I am actively engaged in deconstructing things that do not like my face.