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With a few rare exceptions, flash-based games don't tend to hook the player in for long periods of time.
Kongregate, or any website with many flash-based games, is going to present games which can be completed in thirty minutes or an hour, and there are some that may not be entertaining for more than a few minutes. To compensate for this, flash-based game websites are supposed to offer a huge variety. The entire point is to be able to jump in, play for awhile, and jump back out.
Kongregate passes the basic library test with flying colors, probably thanks to its goal of being friendly towards developers. As of July Kongregate boasts over 16,000 games and rising. More importantly, cross-referencing Krogregate's selection with that of other flash-based game websites did not reveal any serious gaps in Kongregate's library. In fact, it seems that Kongregate often has games that other some other websites are missing.