Nothing really as long as you put the casing/frame for the pollen filter back in.You might get leaves trapped in the heater box which would result in strange noises when the blower fan is turned on.On the other hand if your reason for not wanting a pollen filter is the costs,there are plenty of aftermarket replacement quality pollen filter on the market which is cosiderably cheaper,example of make by Mahle,Mann,Corteco/Micronair which by the way is Honda o/e.