Comparison to Newgrounds
Like Newgrounds, Kongregate has a fairly open uploading system, where authors can add their games to the website with relatively few restrictions. Also like Newgrounds, there is a period of judgement during which a game may be deleted. Unlike Newgrounds though, Kongregate does not permit any Flash movies.
The greatest difference between Newgrounds and Kongregate is arguably the owners. Unlike Newgrounds, Kongregate is owned by a major corporation, and several of the site's features reflect that. For example, Kongregate's PowerUp Rewards encourages users to frequent Kongregate and use GameStop to purchase games.  Likewise, pages on Kongregate link to other websites owned by GameStop.