Hi, Oskar! I've played and enjoyed some of your games, and I wanted to take the time to respond thoughtfully because I truly admire and appreciate people who think carefully about what they're undertaking with an eye toward succeeding at it at the highest level possible.
I think when you say a good HOG game is like a good film or a good book, you are exactly correct. And just like a good film, a great HOG game is actually an entire experience for the player.
GRAPHICS. Obviously, a great HOG is going to have wonderful and wonderfully immersive graphics.
One of the most persuasive scenes I've seen recently is in the Serpent of Isis. Every single time I "go into" the casino guy's train compartment, I absolutely feel like I'm in a smoky room!
MUSIC. As in a film, I think the really great HOG games also have good music in addition to great graphics. The music contributes to the player's overall experience and helps control their mood and keep them interested and engaged.
STORY. While the storylines themselves vary dramatically, if you want to make a great HOG the story needs to be a solid foundation for the magnificent graphics. The story will keep the player interested in continuing to move through the game.
When I played the James Patterson games, for example, I found them to be very much like books, where I played in part just to find out what happens next! And in thinking about it, I think all the great HOGs (Hidden Expedition: Amazon, Return to Ravenhearst, the Serpent of Isis, Agatha Christie, etc.) have the element of anticipation in their storylines.
MINI-GAMES. I think mini-games serve several functions in HOGs. In addition to helping to move the storyline along, they offer convenient and logical breaking points for people who either can't or don't want to spend many hours playing a game at a single sitting. And because the HOG element itself primarily requires one skill (observation), mini-games allow the player to use their brains in other ways (analytical, for example), which makes the game more enjoyable.
Great HOGs seem to have mini-games that are mentally intriguing, appropriate to the storyline, and visually attractive and engaging.
Now for the really important stuff - what distracts from the player's experience and keeps a good game from being great?
LENGTH. When you think about the length of the game, you might also think about it in terms of the length of a book or film. The game should be appropriate to the length of the story being told which, I suppose, should probably approximate an average-length novel.
Not every book is or should be as long as the legendary War and Peace (or this post!) and there are some films that are less successful than they might otherwise be because they're too long. I admit that I've played HOGs that actually fatigued me to the point where it almost felt like drudgery.
I think developers much more frequently get in trouble, however, when they make the substantial investment in making a fine product but, presumably for monetary reasons, have to stop with one that's too short. If the game is too short, word gets out quickly and, if players feel they are not getting adequate value for their money, the developer will lose sales.
Careful planning and budgeting with an eye toward marketing and sales may seem boring when compared to the obviously more fun and exciting artistic and creative elements but, as in great films, they're still really important!
(Another way to go with HOGs, incidentally, are games like the Hidden Object Show and Big City Adventure series, which do not offer much in the way of a story but offer an extremely long playing experience that is economically possible to develop by being repetitive. Others can probably speak to this better, but it's my sense that this type of HOG game has fallen out of favor lately.)
So, in summary, you want it to be long enough for people to feel they've gotten value for their money and that the story has been well told, but not so excessively long that it feels like drudgery to finish (which is rare).
TIMERS. If you've been reading this board it probably goes without saying that a great HOG either offers the player the option to play without a timer or will have no timer incorporated into it at all.
Many people simply will not buy an HOG that has a timer you can't opt out of so, no matter how good the game may be, you'll lose those sales.
SKIPPING MINI-GAMES. The great HOGs generally offer the player the option to skip each mini-game.
Players who are immersed in the game's story (as opposed to those who are playing the game for the challenge of it) may not want to have their experience broken up by having to keep looking things up in a walk-through, so giving the player the option of skipping a mini-game will serve both types of players and help ensure your game's success.
HINTS. Similarly, great HOGs generally have a hint system that's not overly punitive.
LANGUAGE. You mentioned that you are not a native English speaker. I can assure you, your English is FAR better than my Russian and I commend you for it!
Like movie subtitles, if you are going to create an HOG for an English-speaking audience, I would encourage you to utilize the services of a native English speaker or a top-notch translator. If the game uses words that are misspelled or words that are accurate but are not commonly used to describe the hidden object the player is looking for, players will notice it immediately. And just like any small thing can cause a person to make a poor first impression, this can make the game seem to be of lesser quality and/or leave players frustrated.
OPERATIONAL ACCURACY. A couple of operational things come to mind particularly with respect to HOGs.
Players generally do not like it if they have to click an object several times in order for the game to recognize their selection. A click anywhere on the hidden object should be recognized the first time.
Players generally do not like it if they are asked to find a certain item (a drinking glass, for example) and there are multiple drinking glasses in the scene, with only one of them being the correct selection.
Games frequently penalize players for multiple clicks on the "wrong" item and players are frustrated when they are penalized for clicking on items that, despite accurately satisfying the description, just don't happen to be the object the game is looking for.
Hints should be helpful in identifying the object. Occasionally the area highlighted is too broad to be helpful.
Dark scenes set the mood but, excluding the deliberate use of a game flashlight, the scene itself shouldn't be so dark that the player can't find the objects (or at least the objects shouldn't be hidden in the very dark places of the scene).
I've seen this be a barrier in a couple of games, with The Count of Monte Cristo coming to mind as one that had a particular problem with this that I believe was eventually solved after the game was released.
As to why I play, you might find the article at http://www.escapistmagazine.com/news/view/90444-Play-Peggle-Lose-Weight
See, playing games is HEALTHY for you!
Good luck with your work in the future and please be sure to let us know when your game comes out!