It's hard to describe what is missing because the graphics are adequate , the gameplay is adequate, and the challenge is adequate. All in all, this is a modern rendition of the Master of Orion series, and that should be great, but it just feels so hollow. I think the issue with MOO is that is feels like a cash grab. There is nothing really unique in this game. The gameplay, while updated, feels like a rinse and repeat of other space 4X games. The graphics are very space generic. A good example is how the ships are just bland, and the customization for each ship is really lame. The DLC is nothing more than races taken out of the complete game as sold to us as something "new". Finally, as soon as the publishers realized that MOO was not going to be Civilization levels of successful, they dropped support faster than you can blink an eye. The game is so hollow that to get any type of enjoyment you really need to download the 5X mod. Honestly, read the other reviews here, and then go buy a better 4X game.
There is a lot of hours of gameplay here, and with mods (check my post in the forums) there is even more hours to be had. The game requires a good amount of micro-managing of units, but it is not too much to be annoying. Unlike other WWII RTS games, Company of Heroes in particular, the action can at times be slow and therefore missions can take several hours to complete. The game also leans too much towards artillery. So much so, that I joked the game should be called Artillery Battle. I spend a good 80% of a mission just blasting away enemy positions from a distance. I suggest if you do buy this game, to get the AI patch. Without it, the AI is not only dumber, but cheats. I believe in the base game, the AI can see through buildings and maybe other solid objects as well.