Awesomenauts is based on the premise of casual competition. It is designed to be fun. An interesting result of that is the lack of a real consequence when somebody quits your team. He is replaced by a bot that is often more competent than himself and continues to play in his stead. Another player can join mid-game to replace the bot at any time.
Awesomenauts also has a great community (official forum, not Steam). It has a thriving fanart community, a modding one, a tactics one, etc.
This game really opens itself up to modders. It allows creators to modify in-game bots and make their own using an AI editor. Modders can also create gamemodes using this tool. If Ronimo likes a bot, it can find its way into the official game.
Overall, I highly recommend Awesomenauts. It has a thriving community that you should not miss out on. The gameplay is very fun, if less competitive than its other MOBA counterparts. All that is lacking is you.