This is my full opinion:
Terrain is always important, although I understand not everyone can make outstanding environments that make you think "OMFGWTF", there should be at least some effort into this.
EDIT: Also, make sure the terrain matches. Don't have desert orcs in a beautiful lush marsh, or for example night elves in some barren wasteland devoid of life.
Quests, quests, quests. Always have something for a player to do besides grind enemies, or the map could get boring fast. Have things like helping villagers, becoming a part of the city militia for a day, or participating in a hunt for the towns food. And things like warrants, wanted signs, challenges. Have some special boss creeps wandering around in a hidden area, and if you kill them you get a decent item reward. This makes players feel good when they know they're getting something besides a few gold.
A storyline is not completely required, but at least make sure the players know where they're at, maybe how the world was formed, and if you do have a storyline, don't focus the entire game on it. Like I said in quests, have extra things to do besides rush through the storyline. I personally enjoy taking a break from the games story and doing some side-quests and exploring. It's a good thing.
Levels. This can be whatever you like, but, personally, I enjoy having a high level max. 80 is the usual number I would want, but I understand balancing 80 levels is a lot of work and not everyone can do it. Some people disagree, that's fine. Not everyone wants to work up to 80 or 60. Sometimes 40 is enough. Really though, levels are up to you and how much work you want to do balancing them with abilities and items.
Bosses and dungeons are a must for a big RPG, not only for the storyline if you have one, but to have ones that are optional and can be done multiple times for nice item drops. In my opinion, I think it would be good to have at least one for each level group, like a dungeon for players level 10-20, and then a dungeon for levels 20-30, and so on. And not all dungeons should just be storming the place and killing everybody, have puzzles and interesting mini-bosses and creeps to defeat. Bosses, are always my favorite. I'm dissapointed when all I have to do is just walk in with five or six people and DPS him to death. Make it very interesting to fight and kill a boss, have multiple cutscenes for his attacks or dialogue, dialogue mid-fight, battlecries. Give him a variety of interesting abilities, and maybe a puzzling way to kill him. Like having four buildings around him casting a spell, and you have to kill each building in order to lower his health ect. I could ramble on with ideas all day.
Items you should always have in an RPG, unless for some reason you don't need them which is rare, but yeah. I enjoy having sets of items, regular items, everything. Make recipes, blacksmithing, potions, manual enchantments, epic drops from bosses, rare drops from special creeps. I would expect a huge set of items, from level 1 to 80 or whatever the highest level is.
Abilities depend on the RPG really, but I'll talk about both ways. The first is having classes and abilities for each class, which I would also expect a large variety of interesting spells. Buffs, damaging spells, curses, effects, the works. Having a mage, a fireball, some frost armor and a water elemental isn't fun at all, that would just be lazyness. Now if you had a Mage with a fireball, magma summons, lightning from the sky, a powerful triggered mana-armor spell, and a bunch of interesting things would be cool. And thats not even for me, I'm sure many would agree about having custom interesting spells. Now, the other way to do it, would be to learn spells throughout the game. Instead of forcing your mage to learn everything magic, he could learn a priests heal, or a hunters animal taming. Being able to shape your own class is a great idea, but you'd have to make sure it was balanced.
EDIT: Cool Systems are not a must, and it depends on how much work you want to do, but it's always nice to have if you take the time to put it in.
EDIT 2:
Also, I dislike games where one character can walk around and destroy everything. Including other players. I enjoy having to co-operate with my party, like a tank needing a healer, or a mage needing a tank to take damage. Plus I like having customizability like someone else was saying, I forget their name, for example, Gaias Retaliation ORPG, that game is amazing. I really enjoy being able to customize everything on my character. That's a key feature I would love to see in every RPG.
MUSIC: Music is one thing I'll mention even though it's not in the list, in Gaias, they use Oblivions very peaceful music, which made me relaxed and made me want to play more. Music can always improve your map.
EDIT 3:
Interactive town guards, rather than just having a group of people in armor standing at the gate of a town, have guards patrolling the town, some standing at the towns front gate, other lined up defending the barracks or castle/king. And to be more specific about interactive, make your character be able to ask them for directions to navigate around town, if your have a larger city it's easy to get lost and not find things, have your character speak with a town guard and ask for a location such as inn, arena, ect. and then place a minimap ping on the minimap and keep it there until the player reaches that area. This is something I would like to have in an RPG, as I have never played one on WC3 that had this.