There was an article that came out recently that takes a deep dive into on-chain voting mechanisms and how they might be attacked. Its worth a read: http://hackingdistributed.com/2018/07/02/on-chain-vote-buying/

Basically the tldr: The smart contract systems that allow for on-chain voting in trust minimized ways also make it easy to have contracts that buy and sell those votes. It is IMO some what of a profound observation with potentially wide reaching consequences.

One of the primary responsibilities for MKR token holders is to vote on which asset tokens will be used for collateral. The economic incentive to vote is that if bad asset tokens are chosen for collateral, then MKR will be dilluted in order to pay for losses and this will cause MKR holders to vote conservatively. One of the biggest challenges facing pretty much any DAO is voter apathy - basically that there is not much incentive to vote against proposals, and we will often rely on unpopular proposals to just fail due to a lack of a quorum.

In this kind of environment, the presence of a vote buying mechanism can significantly change the economic incentives of the Maker DAO. As a thought experiment, lets say there is a new token called SCAM and a group of people who have a significant amount of this token want to get it approved as collateral for DAI so that its value increases. They create a new SCAM Vote-Buying DAO that accepts anonymous bribes and has a contract whereby MKR holders can receive this bribe payout if they vote in favor of adding the SCAM token as collateral. This is verifiable on chain through a contract, so we now have a trust-minimized way to bribe MKR holders who vote a certain way. We have disrupted the economics of the Maker system to disproportionately encourage "yes" votes. How large would the bribe have to be in order to get a successful proposal through the Maker DAO? We have also potentially created secondary incentives that can create value in the general market - ie. the price of both MKR and SCAM tokens could be impacted significantly by the outcome of this vote.