Well, by choosing this over Write A Major MMORPG, you've shown that you're aware of your limitations. This is good. It takes a huge team of designers and a huge budget to make something on the scale of World of Warcraft, and if you don't have the team (or the budget), it's better you set your sights on something you have a chance of completing. This page, then, will help you figure out just what you're capable of. We'll stick to the tropes and advice that work with a low-budget, low-key, probably 2-dimensional MMO, and get you where you need to be. It goes without saying, but I'll say it anyway: Don't forget to check out Write A Story for general advice on how to make people keep playing. (I suppose we'll get a videogame-specific page up sooner or later.)
Necessary TropesLet's face it: The amount of design necessary to pull off an evolving world is probably well outside your scope. So your world is going to be Perpetually Static. There are a few ways to mess with this, but anything grandiose is going to be in Write A Major MMORPG, so don't bite off more than you can chew in terms of attempted aversions. Death Tropes come in second. Even if you avoid fighting and killing as major sources of experience, you'll end up on the other side of the scale, working with Never Say "Die" and so forth.
Choices, ChoicesMany of the basic elements of RPG's apply, including the need for some sort of stats and level gain. The choice is: What sort of stats are you after? You get to design a system, from the sweet simplicity of BESM's body-mind-soul to the complexity of a dozen or more basic stats, and probably dozens or even hundreds of abilities beyond that. And then there's the question of how you want to gain experience: Mostly fighting, mostly questing, mostly role-playing of some sort? Not to mention, do stats go up automatically with each level, or does the player get a degree of control over which stats get focused on? Or do stats go up only with use? Same question about abilities. Speaking of level — your characters are eventually going to run out of levels to get. When will that be? Level 50? 75? 100? After 4 billion experience points? Never? How far apart will people of different levels be, in terms of power and content? Will a Level 20 character ever see a level 1 character? If a said level 20 character wanted to adventure with a level 35 character, would it be a waste of someone's time? If that level 35 character's younger sibling started playing the game, would the level 35 character be able to do much with a level 1 character?
PitfallsWhile "grinding" in the sense of "spending time gaining levels" is a given, one of the biggest pitfalls is pointless grinding, that is, making players perform pointless quests or waste their time doing things they don't want to do. Some possibilities include "Go bring me that item I could easily fetch myself" and "run around the map a few dozen times talking to NPC's who eventually send you back to me because I had the item / the answer all along". Ask yourself "Would I use a macro program which allows me to grind stuff if that was allowed?". And, of course, Yahtzee's main gripe:
Suggested Themes and AesopsWestern High Fantasy and Scifi are the meat and potatoes of the genre right now. With the rise of Korean MMORPG players (which outnumber American MMORPG players by something around a 10 to 1 ratio — and rising), more and more Asian Fantasy MMORPGs are arriving on the scene. Low Fantasy games are still relatively uncommon, as are Scifi games.
Potential MotifsDashing heroes. Horrific villains. Plots that would make Xanatos flinch. Nightmares from beyond time. And all of them eagerly awaiting the player characters dashing in to stop at the last possible second. Oh, and Loot. Lots of loot.
Suggested PlotsTypically your MMORPG will have some form of overreaching fantasy plot or 3, usually one for each race or kingdom. A civil war breaking out, an old rivalry between kingdoms, a mysterious plague striking the heart of the empire, etc etc. Take a look in Fantasy and Standard Fantasy Setting, and start picking out Tropes. If your MMORPG will have instancing of some kind, start thinking up villains of various scale to populate said dungeons. Deciding how powerful you want your characters to get is important. Newbie characters fresh from the character select screen probably aren't going to be taking out gods, while characters that have been playing for months probably won't be fighting Kobolds. However, there's a lot of things you can do with a little imagination — and the bandits your new players thwart could turn out to be important keys to a Magnificent Bastard's scheme later on.
Set Designer / Location ScoutThis is where you'll get to have fun, but also the place that you could end up screwing it all up. Avoid Patchwork Map. Try to ensure that the map actually looks like it could be a real world, or go the other way and make it as LSD-induced as possible just to avoid any comparisons with reality. But don't stick forests right next to deserts, or make rivers appears out of nowhere. Also avoid Misplaced Wildlife, lest you fall into Artistic License - Biology. Deciding if you are going to have set zones or a more free-range map is also important. Having zones allows you to instance (create pocket versions of content) for groups, this avoids any messy drama with dozens of people competing against each other for limited resources, but also cuts players off from people outside their circle of friends. Having instances also means you can have dungeons be larger on the inside than the outside, depending on how "realistic" you are aiming for, this may be a good or bad thing. It can also allow for more Action RPG style gameplay rather than the traditional open-world MMO.
Props DepartmentGear is going to be your main issue here, although buildings and other scenery certainly count. Your players are going to associate with their gear almost as much as their avatars — indeed, most of their avatars are going to be vastly different looking based on their gear choices. You should itemize your game with roles in mind — or throw that completely out the window and make truly randomized gear. Adding red herrings that do nothing but describe the item to your players, for example mentioning what material a given weapon is made of, or that a shield has 3-4 inlayed gems, adds a level of immersion and will frustrate powergamers to no end. It also will leave options open for adding spells and abilities that affect said items, for example, a boss that is immune to metal swords, requiring players use Obsidian or Stone weapons against him. There are essentially two camps for gear — "Everquest Style" and "Asheron's Call Style", with EverQuest's style being vastly more popular. In EverQuest style itemization, monsters and bosses have very set item drops. Sir Squishybits the Paladin of Cheese is always going to drop a random item from a set table — his Cheese Hat, his Sword, his Shield, etc. This has a few benefits and drawbacks: You can itemize every single item and min/max the risk vs reward — but you have to itemize every single item and min/max the risk vs reward. Players will know exactly what areas and bosses to fight in order to get items they can use, but will also know that once they have gotten all the items they need off the fan created loot lists, they won't have anything left to do there. Asherons Call style loot is infinitely more random. There are very few set drops, instead items are randomly assigned characteristics per a point system. For example, in Asheron's Call, a weapon that drops might be assigned a base type (Sword), subtype (Broad Sword), adjustments (+5 to hit, -5 to parry), element (Fire), enchantments (Heart Seeker 3, Bloodletter 4), activation requirements (Arcane Lore 250), material (Jet), gems (a Ruby and a Sapphire), and mana (1000 max, 5 per minute). Each of these characteristics are given a point value, a given encounter drops a certain amount of gear. This has a few interesting possibilities - you do not have to itemize the gear in your game, save for specific special items. Also, You can tune bosses, treasure chests, and encounters around a points system, allowing players to never run "out" of content in a given area — there's always a chance that a boss will drop something you can use. However, it tends to devalue the individual items over time. A third option is a hybrid, such as what World of Warcraft does. Warcraft has very specific (Everquest style) drops from bosses, with specific drops that appear off certain groups of enemies. However, it also has pseudo-random items that drop off of random enemies, so called "junk greens". Unlike true randomization, these items have a base type based on level and a suffix that gives a very specific set of bonuses based on a point system. Even the set items are based on this point system, with a given item having a certain amount of abilities that adds up to a specific total in points. Warcraft does add a "rarity system" to the mix, higher rarity items of equal levels have a higher points total. This rarity style system would easily be back-portable to true Asherons Call style random items as well. Your game may or may not be big enough to have a crafting system, it depends ultimately on the scope you are aiming for. If your game will have one you should decide early on if it will replace, supplement, or merely distract from other sources of gear. Most current MMO[=RPGs=] go with the later two — crafted gear is vastly under-performing compared to items from dungeons, and outright pitiful compared to raids (big dungeons). Some ways around this include making crafted items take a long amount of time; make crafted items very random; make crafting items require supplies from dungeons; make crafted items only work on EXISTING items (for example, altering items' enchantments or changing them cosmetically); or making crafted items equal or lesser than items with a greater risk-reward — but allow crafters to tailor make items for better use.
Costume DesignerGear choices are pretty simple — if you are going with a fantasy MMORPG, you are going to have leather, cloth, and plate. You can get as fancy as you'd like, or go Strictly Formula, it is up to you. There is a lot of legroom in designing say "Platemail", for example, take a look at the current "Tier" armors in World of Warcraft — while the T7 Death Knight and T7 Warrior gear are both Platemail, they are both extremely different looking. However, going the opposing route — designing armor to look very similar to real life armor — is something that has not been done much (and which opens up interesting merchandise options). Scifi is a bit more complex. Leather, Spandex and Rubber is probably going to be a mainstay, as will be glowing and blinking lights. Outfits and Fashion should be considered when you are building up the backstory of your world, and armor will likely follow closely. Research modern day military uniforms, if your Scifi game has military forces in it, they should have a professional looking uniform to match. Remember that most gamers are male, but this stereotype is changing rapidly — Stripperific outfits may not be a wise choice. Consider developing your gear around the idea that you will allow it to come in any color you want (or one of a set number of colors), perhaps as both a primary and secondary color. This will make adding Dyes to the game much easier, and giving players choices is never a bad thing.
Casting DirectorThere's a lot to say in this category, but let's start with a personal complaint: Why is it that female characters unfailingly possess large breasts? Could you think outside the box of D+ bra sizes and consider that girls might want to see a more sensibly proportioned heroine - or even one who's flat-chested (though decidedly not male)?
Stunt DepartmentLicensing a big name graphics engine is an option, but probably a poor one — licensing such an engine will likely eat up most of your budget. A homebrew engine or one based on an open source engine (for example, Quake's now GP Led engine) may be a bigger return on investment.
The Epic Fails