I hope Jeff is asleep this time around....maybe I can slip this past him :^)

Good infills boast a tight mouth, but that can be had with a Stanley if you
want to futz around with it long enough. The 3/16"+ thick irons are great,
too, but you can get thicker than normal irons in a Stanley. Most infills are
thicker, though.

The real clincher is that infills offer a rock-solid bedding area with a
beefy lever cap and cap iron system that can eliminate any possibility of
chatter. This, coupled with the thick iron and tight mouth, is what makes them
the ultimate smoothing machines for difficult wood. 

Even so, you'd be better off getting a Stanley smoother first. If you
regularly use figured hardwoods that the Stanley can't handle, you can splurge
for a good infill later on. You should have a Stanley anyway, for preliminary
smoothing and knotty lumber, to save wear and tear on your infill iron.

I keep my Norris sharp as possible, and use it for the last few passes after
first smoothing with a Stanley to remove marks and major tearout from the
jointer or jack planes. I also try to never run it directly over knots, so it
doesn't get small chips taken out of the edge, which can cause small streaks
to show up in the work. Infills are the Terminators of smoothing planes, to be
called into play for the final cleanup.

Ray