Why aren't there any aliens knocking on our doors?
There's a neat argument that runs: if all of humanity's previous evolutionary/development steps were relatively easy, but we haven't seen any aliens, then either 1) At least one of our assumptions is broken or 2) It's extremely difficult for a species to get off of their home planet.
Aside from being a lot of fun to think about, it's biggest flaws are: 1) it's assumption that the Intergalactic Max Speed > Speed of Light, and that 2) We are a compelling target. ;)
Space is really big, so if you can only go 3E8 m/s anywhere, you're probably going to automate a slew of the exploration, and on top of that, only send those automated probes towards things that look interesting. And then after confirmation of an interesting find, then send something that would prepare that place as you saw fit (Genesis Nuke anyone? ;).
Let's restate the argument with more solid grounds. If all of our development steps were easy, and we've not seen any aliens, then one of following must be true:
20061203 update -- since the max speed of information transfer is 3E8 m/s, you'd probably make probes self-sufficient, i.e. they'd include a Genesis Nuke and something that would signal back to others that a planet had just been nuked. And ideally, the probes would be self-replicating.