Don't be a Mary Sue. A Mary Sue is a supremely over powered character with no flaws whatsoever. These characters are not interesting and are never fun to be around and generally hinder the RP community.
Don't God Mode. God Mode is when in an RP combat situation (If you do that) a character is never injured, but can maim, butcher and kill others without harm or the other character's consent. Doing this
will often get you shunned from any decent RP community.
Don't be a Special Snowflake. It is good to make characters unique, but some people go too far. Being a part-Werewolf, part-Vampire Hagraven with glowing red eyes is not a character most want to hang around with. The same goes for characters that are related to some major NPC or have extreme political power or high Noble titles.
Respect the setting. We play in established universes because we like them, so naturally if we see someone in the Elder Scrolls universe RPing as Aquaman (Though I don't see why anyone would want to). It can be annoying for those of us trying to remain within the setting.
Don't bend the lore. The Sload, Falmer, Dwemer, Kothringi and many other races all exist, but RPing as one would be very snowflake-ish. The same goes for being something like a Dragonborn or understanding how CHIM and other higher secrets work. Just because something exists in the setting doesn't mean that it is fair game for a character.
That's all I have for now, feel free to add on.