Oh, and one other thing:

6) Combinations - For any game utilizing items (which is pretty much every RPG, but also many FPS games), there should be ways to combine those items into something better. Something that is more than the sum of its parts. Breath of Fire III, for example, has 18 'genes' that can be combined up to three at a time to mutate the character into various types of dragons. There are only a couple dozen outcomes (despite 987 combination of up to three genes), but it's still a really cool idea. Final Fantasy VII's materia combinations are also rather cool.

Why don't they try to keep the balance

Ok, you are right when you say that you can't make the action too non-linear. Obviously, you will end up trying to understand a story so vague, that you'll lose interest for the whole damn game. But you should be able to make, for example, an alternate route in some walls, or blast your way through a door with a rocket. After all, if you have explosives powerful enough to take an objective down, why don't you use that to make a way through all the crates which inevitably block your way in every FPS?
I enjoyed playing Syberia 2, for example, but from time to time, I also play Counter Strike Source.
A game just has to be enjoyable, has to have some humour, some graphics, and a good soundtrack, if possible. Also, the action has to keep you interested in what will follow next.
A good example of a playable game is Warcraft III (both of them), which can be played several times, and it's still fun. Another fine example is Brood War, which has that "je ne sais quoi" that keeps you with the hand on mouse.
Maybe a good game just has to be made more "for your soul" than just for the money. When you really strive (as a producer) to get that warm feeling in the hearts of the players, that's it: You and your game will be remembered.
This applies, in my humble opinion, to Half Life 2, and to most of the Blizzard games.

And despite some people's outright hatred for it, I rather like the Steam interface for managing your games.
If you are building a game which can have mods and add ons, it only makes sense to build a "wrapper" around the "main" game so as to keep track of, and launch, particular mods. Building into the system a way to validate files to make sure they haven't been ruined by upstart modders. You could even have such a thing as a "mod adder", so instead of having your users screw around with your folder structure trying to dump files here and there, you build an add routine to install mods automatically.

So there.