DOS attacks are easy - just make tons of bogus transactions in super-size blocks. Other miners will discard the block immediately for being too large, but this client will be stuck verifying all the transactions. That attack doesn't require anyone else to agree.
This still exposes you to fake confirmation attacks. Any miner can give you a block that follows this clients rules and as long as most miners aren't following these rules, he can guarantee the block will be overwritten (reorged out). This allows cheaper double-spend attacks.
Actually around 75%
51% is guarantees that over infinite time consensus will be achieved. 75% is just a stronger guarantee that it will with higher probability come to consensus in a shorter time.