Sensor dust is the worst. I switched from shooting on Canon DSLRs several years ago and now shoot on Sony mirrorless - the NEX-7 and a7R. Because I exclusively use primes I tend to swap lenses quite a bit and as a result my sensor gets dusty fairly quickly without a mirror to shelter it. Even with a blower bulb it tends to show up. This isn't much of an issue because I also like shooting wide open including down to f/0.95 and sensor dust is never an issue when shooting fast, but then I'll decide I need to go to f/11 or slower for a landscape shot and boom, I've got smudges in the photo thanks to the dust on the sensor.