I'm Grace Note, a community manager at Stack Exchange. That said, the below are my personal thoughts.
I don't think emoji would be practical for implementation, for a few reasons.
- Emoji don't have a consistent schema. Does
:) mean someone who's cheerfully making a simple smile, or is it someone who is excited with a huge tooth-filled grin? Do you need
:-) to actually make a smile, or does it become something like
:smile:? Is it
(:? That varies based on what system you're using for emoji. Making these decisions is fine and dandy if you're designing a system with the intent to support emoji, but we don't have that prerequisite at the start here.
- Emoji would be bulky. We don't even want to turn on MathJax on every site because of potential slowdown issues. One thing I can very warmly recall of forums is how long it takes to load the post editor because of all the emoji. So imagine how much we'd have to deal with that once you start introducing that lot on pages.
But perhaps, most of all.
Emoji don't really fit the system we have here. Emoji are designed to be friendly, chatty, and communicative of emotion. That's why they're pretty fancy for chat clients, forums, and other systems where you have people who are engaging in conversations with each other.
Stack Exchange isn't a conversation engine though. We even discourage greetings, gratitudes, signatures, even +1/-1 in comments, because of what we call "noise" distracting from the "signal" that is the Q&A content. Even comments on posts, the point of comments is to inquire, clarify, or expand on the content of the attached post. A smiley here or there adds a little color but importantly does not currently distract from the content therein because it is innocuous and uncommon.
Start making them have graphical representation, and it'd increase both their visibility and their frequency. You'd skim a page of words and words but BAM smiley, and that stands out and brings attention when in the long run it's the least-deserving-of-attention item on the page. Meanwhile, once you've got built-in support, it implies that these are tools meant to be used, which would most likely leave to a rise in the usage of emoji. Which means more noise.
Emoji are fun and an amusing system. Especially in places where they have particularly creative and expressive choices, their over-use often gets overshadowed by the fact it still makes me smile (or otherwise react appropriately). We're simply not the right kind of system for it, though.