What about this: Click to place one block, hold and drag to select the area you want to fill, release to fill out, right click to cancel. That\'s even faster. It should not replace any blocks that are inside the selected area by default, maybe hold down shift to ignore those blocks and replace them. Same hold and drag stuff for deleting (left click to abort), delete all blocks inside the area by default, or delete only the type of block you have currently selected by holding shift.
While we\'re talking about speeding up building: How stairs in Minecraft rotate depending on where you click would be great to have for wedges and corners.