2. Screenshots or graphics must never reveal a part of the game that the player has yet to experience.
If you're making a survival horror game, do not show your players an image of a monster they will not encounter for another 40 minutes. If you're making an open world game with an emphasis on exploration, do not show them a screenshot of a beautiful location they have yet to discover for themselves. A bit of consideration for the content of the loading screens can preserve some powerful game moments. Carelessness will spoil them.
3. All loading screens must contain a progress indicator.
Respect your players and provide them the courtesy of showing exactly how long they have to wait for the game to begin again. Psychologists and usability experts have already done the research, please recognize their work. Just look to your OS -- a looping animation of indeterminate length on the mouse cursor is only used for very short tasks, while those that take more than ~4 seconds require a progress bar. Be creative with this progress indicator. It does not have to be a bar. Any kind of animation with an immediately definable end point and steady progress can be used by the player to estimate loading length (The loading screens in the Halo games are excellent examples of creative progress animations). If the short length of the loading does not warrant a progress indicator, you should not be using a loading screen at all, but a loading indicator overlay on the paused game world.
4. All loading screens require a prompt at the end before transitioning back to the game.
If your game cannot be paused while it is loading, then you must treat the loading screen as a pause screen. This is a basic usability issue. Players do not want to physically leave the game if there is a possibility of it beginning without them, so they will wait for it to start and then pause it. Their time is being wasted; you are potentially forcing them to sit through a loading screen without even the excuse of generating ad revenue, let alone a meaningful interactive experience. Give control of the transition from loading screen to game to the player. Require a key press once the loading has finished.
5. Make sure any text or audio tied to the loading screen is not cut off prematurely, and provide alternate ways to access it. If the loading screen must include text or audio, such as a mission summary, then make sure it can be fully read or heard before the game begins. Do not rely on an average load time to estimate how long your sound bites or wordcount can be. New hardware (whether in the form of an upgraded PC or newer generation backwards compatible console) will reduce average load times to the point where players will only be able to catch a glimpse of the loading screen content. This point should become obsolete if you follow Rule 4. There must, however, also be a way to access any important information made available on loading screens through other means, such as a journal or similar section of the in-game or pause menu.
6. The loading screen is not a suitable place to put hints or tips or control reminders.
If you have to remind the player of the game's controls during every available moment of downtime, it's an indication you need to go back and rework your tutorial section. If you find it necessary to provide unprompted hint information such as how to efficiently fight enemies or use basic equipment, you need to go back and rework your entire game design. It is not an excuse to say these are for players who have been absent from the game long enough to forget -- your loading screen likely only contains one randomly chosen hint or control reminder, hardly enough to fully reintroduce the player to your game before they are dumped into it. At best, loading screen hints annoy players already familiar with the particular mechanic or dynamic. At worst, you're ruining the game by denying the player the opportunity to discover these things for themselves.