Usually, this means "play a great game and see what makes it great." Sometimes it means "play an absolutely horrible game just get a list of things to Never Do." (This is why Daikatana is on my list.) Once in a while - and these are the really painful ones, even more so than the horrible games - it means "play a game that falls short of being great, and see what they did right/wrong."
Lately I've been playing Dark Cloud. It's a fun game. It's not a great game. Basic plotline: There's this big evil genie, brought back from his imprisonment from some guy that I swear is supposed to be a German war leader, and he's tried to blow up the world because, hey, he's evil. Luckily the Fairy King (no, not making this up, just keep reading) has saved every individual building and things inside the buildings in spheres called Atla. For reasons unexplained, he's picked you to go find all the Atla and reconstruct the villages.
Why is this German guy all military, while about 95% of the world is apparently rural? Is it part of the same kingdom? Are they different kingdoms?
What's up with the Fairy King? Is he more powerful than the genie? If he is, why doesn't he just squash the genie? If he isn't, why doesn't the genie just squash him?
Don't I have to do *anything* to earn the Fairy King's trust?
Why doesn't the genie just blow up the towns again once you've rebuilt them?
On to gameplay. A lot of games take the position of "do one thing, and do it really really well", and this gives us things like Doom 3 which has great graphics. Oh wait, I'm supposed to be talking about gameplay. Well, okay - take The Sims instead. For all the problems I have with that game, the fact is that it managed a rough abstracted representation of day-to-day life really well. This is a fine way to go about making a game.
Some, much rarer, games take the position of "do everything" - look up Wario Ware for an example of this. This genre isn't even close to mature yet so I'm not going to talk about it anymore, there's nothing meaningful to say for another few years.
Some games, with enormous budgets, take the position of "do a lot, really really well", and this works fantastically if you're guaranteed massive amounts of revenue. Which some games are - just look at FFXI, or WoW, or EQ2. It's no coincidence that MMORPGs tend to dominate this area.
Some games decide to do many things, and don't have the budget to do them very well. Usually you end up with a "core gameplay" and a series of minigames - Ratchet&Clank with its dogfighting and turret gun, for example, or Jak II with *its* turret gun (turret guns are popular for this), and so on and so on. Dark Cloud, unfortunately, doesn't have a core gameplay. It's got two core gameplays. There's the section where you build your village, and the section where you fight. Neither one is particularly well-designed and both are critical to progressing. To make matters worse, it also tries to be an RPG with a lot of emotion (which is very rarely done well with modern games - to my knowledge, this hasn't been pulled off successfully since the FF7/Fallout era.) And if you want to get even worse, it's got a minigame that can basically be described as fifteen seconds of DDR.
There's one more big category, I'm discovering - Extras. Things you don't need to do to "beat the game", but are still there to be checked off if you're obsessive like I am. Ratchet&Clank did this marginally well with its Skill Points - extra challenges you had to do - but with few exceptions, they didn't give you enough hints to discover what you had to do on your own. Jak&Daxter did this brilliantly in that you basically had a checklist of which areas weren't complete, and what needed to be finished or found. The king of extras is, as far as I'm concerned, Donkey Kong Country 2, which had a metric ton of random findables in each level and very good help to tell you what zones you hadn't finished.
Dark Cloud falls flat on its face with this. Yes, there are extras - but aside from the Atlas, which are easy to find and basically all required, the only extras are weapons. And there's no way to find the weapon upgrade paths - in fact, you can't even pick which weapons you want without just getting lucky! Add this to a highly complex weapon upgrade system, and without spending days upon days of research, you're pretty much restricted to finding a FAQ or working yourself into a corner.
So to summarize: I could finish the game, but there's no good plot. There's no good action. And I can't even get a feeling of accomplishment because there's no way I'm tracking down all the utterly random items I need to find.
And to make matters even worse - the levels are all random. And nearly identical. There's basically zero gameplay variety.
Time to shelve this game, methinks. Now I know a few more things not to do.