If I roll on a nearly-full server at launch, I'm likely to be able to have guilds to join that will be into WvWvW... but there may be too many players queued up to get in very often.
If I roll on a medium-low pop server, I might run the risk of not enough people or guilds playing WvWvW, which might cause a feedback loop of getting stuck in loser-tier matchups and noone wanting to play it... I worry a server may then self-identify itself as the "We suck at/we don't WvW".
High population. There are four WvW maps, and I have yet to see all four on a full server be full. If the one you want to be on is full, you can enter a queue and continue to play wherever you currently are until your turn comes up. You aren't forced to stare at your monitor twiddling your thumbs while waiting.