Straight to the point: every game is educational, but ones are more educational than others and in different aspects.
For instance people can learn a lot about weapons with some realistic shooters, which may be useful for the army and pretty useless in many other cases. If snipers were more realistic players would require to learn some concepts about physics, how wind affects the bullet, etc. Driving simulators may help to learn to drive, this is already happening to some extent pilots use simulators before risking valuable planes. Even Guitar Hero could help to learn to play the guitar (a real guitar), if it was more realistic.
If games were more realistic they would be more complex, arguably less fun, but the point is to have the right learning curve and difficulty ladder, so that it is natural to increase in realism and difficulty step by step. Games like Demon Souls proved gamers like challenges and games like Starcraft proved that gamers can get to be fairly competitive and skilled on games, up to the extent of being considered "proffesionals", and is a game considered to have a quite serious cognitive load.
Without being realistic, games can teach a lot, Theme Hospital is perfect for queueing theory, games like Caesar and Pharaoh give valuable lessons about operations research, and the economy in Eve Online is a simplified version of the real economy, so anything learned there is extrapolatable in a very straightforward way, even if it happens in a completely fictional world (SciFi actually).
Every game is educational, every game presents a challenge to overcome and requires the gamer to learn something to overcome that challenge. A game has to be fun which is a combination of feelings like challenge, power, freedom, achievement and reward. Games are fun and addictive because of this sense of achievement, because the gamers feel they are learning something. Learning is addictive, when a game is mastered and there are no achievements ("official" or self-motivated) left to obtain it becomes boring, usually it becomes boring as hell and the gamer seeks for a new source of challenges, something new to learn and master.
Games are fun as far as there is something new to learn and master. Education is basically about learning. They seem to be perfect allies. Why are educational games a niche full of games that don't seem any fun when compared with the best sellers? On educational games the stress seems to be on the educational part, and not on the game part. A game has to be fun. Educational games are usually constrained by a syllabus, they focus on presenting stuff in a clear and educational way, which is good, but not the nature of a game.
Every game is educational, a point that those making "educational games" seem to be missing, they have to, otherwise their whole model would collapse if every game was an "educational game". The focus is set on trying to make education fun, trying to make a game out of it, but it is from the same old perspective of the knowledge that is to be conveyed, not on the game, the challenge or the use of this knowledge. Paradoxically, educators are meant to make everything easy to understand, not challenging, thus the game becomes boring, with this the game fails as a game, and without emotions the education fails as an aid to learning, because the best way to make something memorable is to attach an emotional load to it, it is well known there is a strong link between emotion and memory.
There is no need to make "educational games", because every game is educational. The point is to make games that are "more educational", therefore the shortest and easiest path is to make "regular games" that are "more educational" than the games we see nowadays. Trying to make a game out of education is (more often than not) failing and a paradox. Some games attempted this in the past to a greater or lesser extent, for instance the Fable franchise explicitly attempted to teach some lessons about morality, which is a complex and subjective matter. However, this is not the norm nowadays.
For instance, something applicable to most games and fairly simple would be displaying some numbers. This would allow the gamers to improve their algebra skills instead of trying to guess which weapon should they pick for which enemies, and the guessing option would still be there for those that don't want to make the numbers. A genre that shows a great potential is strategy games, either real time or turn based. In the case of historic strategy games, like Civilization, this could lead not only to learn knowledge that can be used to beat the game, but also related historical events. Every game has a story, and the story could be part of the history, as simple as that.
Personally, I hope there is a paradigm shift in "educational games" and developers and educators focus on transforming regular games into games that are more educational, because there is a great potential to be untapped down that path, IMHO.
What's your opinion about "educational games"?
Note: Post motivated by an article in mashable.
PS: Bonus: I am improving my English vocabulary with Bioware games, which are not dubbed to Spanish. BTW: Sorry if my English is weird.