The examples on the GitHub repo (like this basic one: http://vizicities.com/demos/basic/) use an OpenStreetMap tile endpoint provided by Mapzen and so will work anywhere in the world. The caveat being that the OpenStreetMap data varies in quality around the world. However, in that case you could use a better quality GeoJSON-based dataset for the area you're interested in if it exists.
Countryside rendering depends on what you want to see. You may not see many buildings but you'll see the underlying map on the 2D basemap and you can always decide to output (or ignore) specific things like roads or fields or trees. This obviously depends on whether that data exists and exactly how you want to output it.
By default ViziCities can't do anything with LIDAR data yet but it's on the list of things to add in the future.
There will be soon, yes. Right now the ground is flat in ViziCities but there are plans to add 3D terrain, though it's not a simple feat. It's possible to extend ViziCities to do this yourself.
As someone recently new to Geo visualization (and data viz in general), I have to say this project looks awesome! Kudos for all the hard work put into this
It's a bit slow on my machine (the linked demos), is that a general issue due to 3d rendering capabilities, or is just the data source for the demos slow?
The performance could be a bit of both I imagine – a computer with a weak GPU or a visualisation at sufficiently large resolution will strain the framerate, especially the higher quality demos (eg. "All The Things").
The data sources are remote and so you may notice tiles and data taking a little while to load before popping into view. This is something I'm looking to improve with more sophisticated networking and Web Workers for background processing without locking up the browser.
I've no experience of Worldwind but Cesium and ViziCities both use WebGL and both display the same kind of data. Cesium is built around the concept of a round, globe-like representation of the world whereas ViziCities is a flat representation of the world. ViziCities is arguably simpler to use and simpler in functionality, whereas Cesium is built with scientific and incredibly accurate scientific data visualisation in mind (eg. satellite positioning).