Poor documentation

Even though you tried to document the game, you missed parts that you knew by heart but anyone else would have to learn. Just because you know that the space bar makes the character jump doesn't mean everyone else does.

Unbalanced game play

The high score list is laden with super high records that no one can even touch without spending hours playing the game. This happens when the creator fills the list up with his/her own scores from testing. Since programming games involves a lot of testing, developers tend to get very good at their games. This can give them a false sense of difficulty and make the game near impossible to progress through.

Bug filled

The more testers you get, the more bugs will be found. Simple as that.

Not fun

You may be a very special developer who's interests just don't match those of others. The game may be very fun for you because it evokes a certain memory or emotion but to someone else, its just another puzzle game with ugly tourists to stare at once the pieces are plugged together.

Missing pieces

The worst symptom of any virgin game is missing pieces. Believe it or not, some people make games or demos and forget entire sound tracks because they've been anticipating the day that they make one for so long, they start to visualize the sounds as they play. In their minds, they hear a click or a pop, or even a song that's stuck in their head, and they subconsciously deny putting any sounds in the game before releasing an alpha or beta build. This is not so bad, but don't forget to mention that there are pieces missing. Also, when testing a game, it's nice to hear the proper sounds and music to get the overall mood of the game.

Testing games on your own is ok if there are no testers around, but make sure to put yourself into other peoples shoes and think outside the box.