I do remember coming across a site that was dedicated to creating a complete database of all game mechanics at one point, but I've forgotten the name and my google-fu is failing.

These aren't exactly what you're looking for, but might be of interest:
http://tvtropes.org/...VideogameTropes
http://www.giantbomb.com/concepts/
http://www.designers...ie_database.htm
http://www.theinspir...ster%20List.htm
http://www.squidi.net/three/index.php

It wasn't the game design patterns wiki, was it?

http://gdp2.tii.se/index.php/Main_Page

Thanks for the links! They're exactly what I've been looking for in my other research. All of my own searches for game concepts, game mechanics, game features, etc... failed me miserably! Stroppy Katamari's suggestion of looking in "game patterns" brought out a lot of great material.

[quote name='Stroppy Katamari' timestamp='1331595940' post='4921510']
Björk, Holopainen: Patterns in Game Design (2005)

THANK YOU! This is exactly what I was looking for. I don't have a full copy of the book to read yet but just having the term "game patterns" to search for has uncovered a lot of the information I needed.
[/quote]You are welcome. While the book is obviously not specific to "2D top-down action-adventure", many/most of the patterns would be applicable to at least some games of that type. It's not a recipe collection, but a catalog of the bigger picture and relationships between families of game mechanics. I suspect it would be most useful when you have at least some ideas floating in your head so you can use it to find complementary elements, or with a more complete design, to figure out what needs to be replaced when things somehow don't seem to fit together.

One of the writers was a friend's colleague.

God, what a whiny, needed bitch.

How about you do your own research instead of asking a community to do it for you. Geez.

God, what a whiny, needed bitch.

How about you do your own research instead of asking a community to do it for you. Geez.

Anyone wanting to respond to this please just ignore it.

Also, "start from the basics?" What basics? Are my basics and your basics the same?

There seem to be some warped perspectives on using authoritative resources here. Like, "figure it out yourself in the dark and it'll be that much sweeter." Sure, you can learn to paint or play the guitar without ever cracking a book or taking a lesson, but you would WASTE SO MUCH TIME. Doesn't game design take enough time as it is? There's a lot of what I call "go with your heart" advice happening here. Advice that pop-philosophically questions the relevance of the initial question itself without working to add anything of substance to the discussion.

Brainstorm what kind of npcs and objects would appear within the setting/theme of any given level.. What's naturally occuring? Man made? Friend? Enemy? Whatever other category you feel is worth adding!

Come up with various ways they can interact with each other and the player.. How can be they hazardous? helpful? What's inherently interesting about them? Settle on the gameplay mechanics they add..

It can be helpful to look towards other games to help with ideas, but if they're your main source of inspiration.. your game is a lot more likely to be derivative..