It would be very l33t if you could search through your old entries (or through someone else's) by mood. For example, if I wanted to see the last time my friend kat felt sad, I could do a search for 'sad' and see when it was.
* I'm not sure since I don't know exactly how they program everything, but it theoretically shouldn't be *TOO* difficult to put together.
* It would make things more convenient.
Like I said, I know nothing about how things are implemented on LJ, so I can't really put much here. o_o
I just thought it was a cool idea. :)