Daniel Cazzulino posted a rather interesting use for Expression Trees (he says 'Linq', but it's not really Linq, it's one of the features born out of need from Linq: Expression Trees).
I've had a couple run-ins with situations that this could've easily solved and I wish I had it back then.
If you have a spare moment or two, check it out, it's pretty cool.
The expression tree and delayed execution thing, while very helpful and useful, is a little bewildering at first. I'm familiar with this stuff and I had to do a triple take before I understood what was going on.