Players will all have pads, and these will be save zones as to prevent any sort of griefing. So you can't be a dick by shooting up someone's home or stealing their cars. Plus, other players have to be invited in first before they can enter your pad.
Rockstar's official site also details how the microtransaction stuff works, but to sum it up it will all be fair. "To allay some concerns we’ve heard on this subject, we can assure you that the game is designed such that the option of purchasable GTA$ should not disrupt the playing field." No one can start up GTA Online and just start spending real money to rack up their own skills as everyone will have to rank up to their Reputation Points first.
Note that the Content Creator toll won't be available right away. It'll be released later this fall, so you won't be able to create your own custom races or deathmatch jobs just yet.
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