I agree, that actually surprised me a bit. With a real SLR, some adjustments are immediately obvious. When you change the aperture, the view becomes brighter/dimmer and the depth of field changes.
Some, like shutter speed, don't make a difference until you hit the button, but it would have been nice to see the DOF changes in real time.
On most DSLRs, I believe, you have to press a DOF-preview button (whatever it's called), instead, it remains wide open making it easier/possible to accurately focus using the viewfinder. Focus is what I miss most in this simulator, it leaves out a very important part of taking of the picture…
And of course, there is virtually no way to reflect the ISO or shutter speed setting, using the optical viewfinder. Not a bad idea to "simulate" here as well, because it's not funny to shoot in the aperture-first mode and forget to reset ISO from some ridiculously high value that you had to use yesterday in the evening.
On the other hand, I was confused and disappointed that the lighting slider in the simulation doesn't do anything (well, it shows an icon, for a while, couldn't it just dim/lighten the scene a bit?).
On most modern (D)SLRs the lens isn't stopped down to the aperture setting until the shutter button is depressed (or you activate the DOF-preview button if it's available). Even when the lens is stopped-down to the aperture you've set, the in-viewfinder effect is far subtler than it ends up being in the actual exposure.
Some, like shutter speed, don't make a difference until you hit the button, but it would have been nice to see the DOF changes in real time.
Still, a great little tool. Very handy.