Kind of small but wouldn't mind some clickable JS assists for inserting markdown link URLS and images .. or just a quick and dirty markdown assist cheatsheet that on-click pops up below the textbox like reddit has (rather than having a pop-up to an outside URL in new tab). Maybe a similar emoji insertion assist for non-mobile users.

+1 vote for top of the day/week/month/year/all-time - one of my favorite reddit features.