I've seen a lot if talk of town/city size in terms of environment, but what about population? It'd be great to see some more people walking about and in taverns and stuff

It would, but an individual TES-class NPC with scheduling and AI and such is a relatively expensive being, too high a population density would cause combat in that area to grind to a halt on consoles. It can be higher than oblivion simply through oblivion's lack of optimisation, but there'll be no crowded cities.

I'm sure they're gonna make the cities approriately big. Sure, the cities in Oblivion were small, but the populations always fit the citysize. Well, maybe the imperial city seemed a bit underpopulated, even for that small size. Though, I'm sure they're not gonna have a lot of emty houses just to make the city seem bigger.

They created "filler" one-liner NPCs to help fill the void that was present in Oblivion. The Gamebryo engine just wasn't strong enough for the desired population, given the circumstances. It worked out fine in Fallout (after all, the population is going to be smaller after the apocalypse)
Thank god for the script extenders, they really improved playability and stability on those games (that and script optimizing mods)

It's confirmed that there will be a lot more NPCs in open cities, but some of them won't have dialogue options, which is kind of good because Oblivion, most of the filler NPCs had rumors. Big deal.

I would like to have cities more populated than Oblivion, especially considering that the cities are bigger as well. I also just like to have more characters in games, because it makes the game feel so much bigger than you.

I want cities like in Assassin's Creed Brotherhood. Teaming with life, and while not all of them will talk to you, there isn't that desolate feeling that you have wondering around Imperial City trying to cure your boredom.

Hard to say for sure because there's a difficult balance between an enormous population of simplistic characters or having few characters, but making sure they're all well developed. For one thing, I hope all characters are named, I hated the generic "Tenpenny Tower Dwellers" and "Megaton Settlers" in Fallout. Having characters with only one line of dialogue and no name is a definite immersion breaker, but so is a city that only seems to be at 5% carrying capacity, like Oblivion's Imperial City.