When I developed Epic, the features I added were random and haphazard. First I had a mockup-game of a character swinging a sword, then a shield, etc. After that, I added little HP and MP bubbles. I then added the inventory window, and decided to add the ability to loot the corpses of enemies. This was followed by chests, instanced dungeons, electricity puzzles, and finally quests and partying.
The point I'm trying to make is that I really followed no design document or set rules prior to making this game. I simply just had the vague idea of "Skyrim meets Zelda meets WoW" and I acted on it depending on what felt right to me. In the end, I came up with a very coherent and unique game that is fun to play and doesn't really feel like a clone of anything.
When I want to add new features, I just focus on what users are asking for and what I feel the game is lacking in. There was a strong focus on fun and intuitive customization options because I felt that users would appreciate it(And they do. People love the crafting and enchanting, lol). Things like the bank are being adored by my players and I would have never added them had I not listened to my feedback :) I suppose I never really focus game updates on one specific feature(Like gear dyes, which epic has lol) because my updates aren't based around the features but instead how many levels have been added. In the time it takes Matt to add 4 dungeons(About one week), I have added several other features to the game.
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||

That sounds good, but...
So much for distinguishing this game from others =)
I'd take the opposite approach. Being overly descriptive seems like a good approach. It sounds like a smart thing to say "let's plan everything before making it", but that's not necessary. You're just creating more work. By the time you're done describing what items are and how they can be equipped, you'll be burned out. When it's time to describe what makes your game unique your motivation will be gone.
You don't need to describe the gameplay to someone who knows nothing about computer games. You're describing your ideas to someone who knows a lot about computer games. Take advantage of this. You can skip the obvious details about how many things will work.
Whenever possible, I try to plan things in terms of other games. Instead of saying "combat is a feature of the game", you'd say "combat will be like Final Fantasy X", or "combat will be like Streets of Rage meets World of Warcraft". Often you're just describing the game to yourself, so reference whatever you're familiar with to make it easier to describe your ideas. The features you need to plan are the ones you don't have reference points for (which is worth the time spent planning, since this is what'll make your game unique). Even then, not all games have unique features, so large amounts of planning and detail is rarely necessary.
If anything, BYOND games are boring because they use this approach. People build up simple, obvious details into a mountain of work. I can't find the posts anymore, but there were some blog posts about Eternia's shopkeeper interface and how you could buy items to dye your armor. Those are the kinds of features that BYOND developers focus on far too much. Not only should those features be added later (unless the game is 100% about armor dying) but they should be trivial.