There are several big mental pitfalls that you have to avoid when approaching this subject. Most people manage to get stuck in at least one of them, and consequently never say anything that is even remotely plausible. They are, in no particular order:
Unrealistic Technology Expectations
This idea comes in two closely related forms: "Future technology will be
just like current technology" and "Future technology will be just like old
science fiction." Wrong. This kind of thinking leads you to ignore even
old, modest technologies like robotics and genetic engineering. If you
want a realistic picture, you need to deal with the prospects for
nanotechnology, AI, and superintelligence in a realistic manner. If you
are going to dismiss them, you need to explicity say so (and supplying a
justification for doing so would also be a good idea).
Short Time Horizons
We aren't talking about a few hundred years of event here. Or even a few
thousand, or a few tens of thousands. Unless you think there is some
special reason why technological civilizations have just now become
possible, you need to deal with time spans of at least tens of millions of
years. This is one of the reasons why most of us don't find scenarios that
include technological roadblocks or galactic Prime Directive treaties to be
very plausible.
Insufficent Appreciation of the Effects of Diversity AKA "maybe not everyone will want to invent nanotechnology". It doesn't matter if 99.9999999% of all sentients spend all of eternity contemplating their navels. If I had the technology of 100,000 years hence, you can be sure my personal projects would be visible from distant galaxies. Simple evolutionary processes will give you a universe dominated by active explorers, even if almost everyone starts out being a homebody. This means that if you want to hypothesize a universe full of advanced civilizations you need to think really, really hard about explaining why they haven't set up shop here.
Exaggerated Respect for the Immensity of Space The galaxy is big, but it isn't infinite. A single expansionist species with sublight travel could easily colonize every single system in it in less <10^6 years. Suggesting that we are surrounded by aliens, but they don't happen to have visited us, is equivalent to sugggesting that a particular lot on Manhatten Island might remain vacant for the next million years.
Exaggerated Sense of Mortality
AKA "maybe the aliens did thus-and-such, then died off..." What exactly do
you think could have killed them? Most of the ideas I've heard wouldn't
kill of modern-day humanity, let alone spacefaring aliens. Remember, even
without nanotechnology, a single surviving colony could repopulate an
entire species in a very short amount of time (instantly, form a geological
perspective). If you use a realistic technology projection, and keep in
mind that some small fraction of any population is going to be prepared for
even fairly extreme emergencies, it is hard to imagine any event that could
kill of a spacefaring species. Certainly, any disaster that big isn't
likely to leave any intact planets in the galaxy.
Billy Brown, MCSE+I
ewbrownv@mindspring.com