Hostiles, parties, PvP, weapon classes, and specialization.
Combat has an increased difficulty scale, and engaging with hostiles will equal near-guaranteed death unless properly skilled and equipped.
World PvP, faction combat, dungeons, certain quest lines, and miscellaneous activities will all be available solo or as a party.
Melee and ranged combat, including kill moves and perks, are fully available and you may specialize in any combat category your character desires.
Choose a combat build wisely. You must consider competing builds and you want something that matches your character because you will often only specialize in a few combat and defensive skills over your lifetime, if any at all.