>Are there any imageboards features other than sage I should be aware of?
noko is the only one I can think of. BBcode exists here as well; read the FAQ.
>And lastly, I’ve been wondering if there is any motivation behind rules 1 and 2?
It's essentially profiling. Posters that use *actions*, emoticons, and the like are simply more likely to be shit-tier posters. If you take a moron and force him to use proper capitalization, no emoticons, no actions, no random japanese words, and whatever else, he won't automatically become a good poster but his posts will at least become more bearable to read. And if he chooses to not follow the conventions then it's obvious he has no place here, making him an easy scapegoat.
>On the subject of random Japanese words; what about Japanese words that can't translate into the English language?
Words that cannot be translated effectively from Japanese to English can very well be left in Japanese, so long as it is expected that the audience knows the meaning of the Japanese.
However that is a rule for translations, and very few translations are actually done on this site. While it is perfectly reasonable for Marisa to call Reimu "Reimu-chan" in a doujin, it is not reasonable for you to write "Reimu-chan" in a story simply because neither the writer nor the audience is Japanese (probably). There is no risk of subtle meaning being lost in translation when no translation is occuring.
>Also, using this ಠ_ಠ smiley doesn’t seem to be looked down upon. Is this true? And if so, just out of curiosity, why is it fine using this particular smiley?
These guidelines are simply guidelines and not absolutely enforced rules. The guidelines should only be strictly followed until one has spent enough time on the board to actually learn the etiquette to follow, rather than following the crash course. Sage is a good example of this; it is frequently stated that every post should be a sage if it is not a writer giving an update. This is certainly not true, but it is a good guideline to follow until you learn to not fucking bump year old stories.
For example, if someone posts "lol Reimu was so mean to poor cirno in that last update :(", they'd be a fucktard unnecessarily using emoticons and will probably get called out for it. However if someone else posts "Isn't is sad, Sacchin ;_;", then that's perfectly acceptable (more or less). Why? What's the difference? Well, you just need to lurk for a long time until you pick up on what that meme is and where it came from (e.g. forever, because it originates from 4chan and honestly isn't used that often here)