Put simply, a serious game is one designed with entertainment not as the main intent. Many ways to work that and many different ways to name a serious game but this is a concept that has been around for a long time and a title that is becoming more and more recognized. As a growing industry, especially with newer and more refined platforms to utilize, serious games are on the rise in the current market.
While these games may not be created to dazzle and delight, that does not mean they are incapable of doing so. If America's Army is any sort of example, a game that spawned a number of sequels since its original debut, a game meant for training can be a load of fun. While there are serious games out there created to send a message, I'm going to be focusing on ones that are meant for more in-depth simulation.
One of the first things a player will familiarize themselves with and be introduced to is the control scheme. Even things outside the realm of serious games, genres such as simulators of any variety can have clunky and unintuitive controls to allow for operation of all features the developers have programmed into their version of realism. ARMA is one example where if you are unfamiliar with the control maps, you commonly won't be able to discover all you are able to do just by exploring the common binds associated with other military shooters that have less depth than something designed to give a feel of realism.
Returning to America's Army, this too was the case however it was also created to be easy to learn by spending time in their handcrafted training missions to gain an expectation for what is in store. ARMA does this as well, but it doesn't go over every single facet which leaves a solo player scratching their head or reaching out for help.
All that being said, the modern industry has provided new tools and platforms for serious game developers to utilize. Specifically, I would like to focus on Virtual Reality. VR is, at its core, an intuitive control scheme that is mostly designed to work on muscle memory to perform actions as you would in the real world. This rising platform is starting to gain traction when it comes to serious games as it, as stated just before, gives the use a more natural experience with interacting inside the application space. While not directly, more experience could be retained and put into practice given the field and game's design principles.
What is boils down to is that, fun can be born from natural and familiar controls. By all means make sure your application contains the elements needed for its purpose, but the path a player must take to reach those functions needs to make sense and never be something they have to fight with once they gain competency.
Let's continue to talk about VR as a platform. While there are others out there that might be able to facilitate the application in question, the control schemes attached to those platforms might not be adequate for interaction with the systems or provide a real enough sensation with the experience. This leaves the PC as the main development platform for serious games due to its highly customizable hardware and broad spectrum of capabilities. Attach to this a VR unit and you give your experience a new level of immersion and engagement should you choose to use motion controls along with your head mounted display.
As stated in the last section, natural and intuitive control schemes give the player more experience to take with them outside of the simulation and a faster grip on what is expected out of them for any given action. You remove the need for independent keystrokes and aim for a contextual touch based interaction system that currently can only be found within VR or highly advanced control boards that are made only for a single application. This is impractical for things outside of vehicle and gear manipulation for the most part as a more contextual input can be used for modifications to the application down the line.
A serious game has a purpose to fulfill. To fulfill that purpose usually requires an amount of realism and depth to get the point across. Somewhere, there is a middle ground where needed depth, realism and game all cross perfectly. This is a difficult thing to achieve. The complexity of that marriage of elements aside, the depth of a serious game is an important component. To achieve the desired result that the game is being designed for, an amount of depth is absolutely required to express the experience.
This depth is something that some players will latch onto. If the crowd that actively mods ARMA or follows games such as Elite Dangerous is any indication, it is that depth can create lasting enjoyment within gameplay loops. On the other side of the same coin, needless depth is something that can kill an experience completely and end up with the player not obtaining any valuable information. Maintaining a balance of acceptable and needed depth mixed with a competent gameplay loop is the key to fun and ultimately engaging serious experience.
Capping off the various pieces that could alter the fun level of a serious game, is something that applies to the industry as a whole. Something that has stagnated various genres is a lack of selection and innovation. This doesn't apply too heavily to serious games because of their purpose-developed nature. However, there is still a market for it and some within the same field could be seen as superior to others.
While creating an experience meant to inform, innovation is still key to activating the brain of the player and leaving them engaged with the experience you are presenting. The serious games market is something that needs a little bit of competition within the same sectors of it to produce applications that have staying power beyond simply being the only one available for the purpose in question.
Serious games still rely on the fundamentals of game development to exist and still apply to the critique that is applied to all other applications of similar nature. Serious games can be fun, and there are plenty that have been. There are lists and documentations of these elements and previous successes. This has been around longer that some might be aware of and there is a level of expectation that comes from venturing into certain fields of occupation or topic concerning serious games.
Above are simply my surface level explanations of what I think should be focused on for creating serious games that retain engagement. Explanations from a game design student who will be digging deeper into this topic as time goes on from the creation of this writing. I played America's Army. I had fun. I'd love to have that experience again within a game not designed for it.