#2. Keep it Simple - Chess.com created too many amazing features and products for just one website (you can see many of them here: Site Features and Video Tour . :)
99 Things I Wish I'd Known When I Joined Chess.com

#3. The Track this forum topic toggle lets you follow discussions without constantly checking the thread for replies.

#2 Videos: There is no rhyme or reason to the video section, but it's still fun.
I wasn't even aware that there were videos under FUN -> Videos.
I see that you can sort the Video Gallery by random / most recent / most popular (all time) - BUT there is no search facility or index.

#4 Endless quiz
You can find it at the green bar Fun>Endless quiz
I wish I'd known there's NO WAY OUT of the endless quiz. Once you start, you have to keep on answering f o r e v e r . . . .

#6 The games in the Game Explorer Master Database are not all master games. All the games may be from Master Tournaments but many of the games are very low quality.
Take this game for instance. http://www.chess.com/games/view.html?id=2639737 These are two French Class B players and at least one is a junior who played in the French FIDE tournement at Saint Chely in 2003. The move 3 ... e6 is quite suspect and not likely played by any masters. Even if it is, the move 5 ... Ne7? would never be as it immediately loses a piece to 6 d6. According to the Game Explorer, 21 players played 5 ... Ne7. In response, only 12 players played 6 d6. It is obvious that from this one line, at least 33 players are not master calibre. My experience with other lines says many many games are of inferior quality.
While the Game Explorer is useful, following lines blindly, especially the less frequently played ones, will leed you into trouble. You need to be careful about the lines you choose. You still need to use your brain for every move you make.

That is true of all chess databases. Very useful but use with care!
#7 This site should come with a chess health warning. I am playing many more games than I should (9 is a comfortable number) but I still enjoy it.

#7 This site should come with a chess health warning. I am playing many more games than I should (9 is a comfortable number) but I still enjoy it.
I did suggest a scheme of voluntary restraint http://www.chess.com/forum/view/community/voluntary-restraint

I think I am beyond voluntary restraint. There should be some system whereby one's relatives and/or friends can issue some kind of request for the person to be restrained forcibly! That is what my wife has ordered me to say.

There's some here - http://www.chess.com/echess/game.html?id=9284587 It's actually rather good. You should try it sometime.
99 Things I Wish I'd Known When I Joined Chess.com
#1. Game Explorer - (the Openings Database) - is available for use in Turn-Based chess (though not LiveChess). It's easy to access (LEARN -> Openings -> Game Explorer); it's easy to use; and it can guide you through the opening (typically half a dozen) moves flawlessly. It also identifies the opening, enabling you to become more familiar with them relatively painlessly.