If 20% of bitcoin miners 'wasted' block space with a very high minimum transaction fee to drive up average transaction fees you get a huge problem. (This could actually be a net gain depending on the demand curve.)
If you look at the actual block chain there are many empty blocks so this is less theoretical than you might assume.
Another attack is if you get say 40% of miners to block transactions for a specific company you could create a lot of issues for them at minimal costs.