PB) Taxing more valuable land at a higher rate is a progressive tax. (PRA progressive tax is a tax by which the tax rate increases as the taxable amount increases. A site value tax applies the same rate to all site values.
In (geolibertarian) principle, a site value "tax" is not a tax at all, but rather the recovery of the site's ground rent: the excess advantage you get from exclusively holding that site due to its location in the community and its natural productivity, compared to sites at the margin of production. Sites (as opposed to improvements thereon) are not created by human labor, and so may be held/occupied but are not "owned" in the same sense that created capital is owned. The mere act of claiming and defending a site is not an act of creation, but rather an act of appropriation. Everyone has the same natural right to access the Earth's surface, and those who exclude people from the most preferred or productive sites should pay in proportion to what they are appropriating from those they exclude.
It took me a long time to come around to geolibertarianism, mainly because I thought it implied you could be taxed off the land you occupy. Then I realized that geolibertarian principle doesn't require you to vacate the land. It merely requires that you not try to reap or grant a speculative windfall (in capitalized ground rent) when you try to transfer it to somebody else. So you can keep farming in the middle of Manhattan for as long as you like, but you don't get to pretend that the skyrocketed value of your site came from your agricultural efforts. :-)
PB) It creates disincentives to improve the quality and productivity of the land, or of a human being (human capital). (PBA site value tax explicitly excludes from the tax basis all improvements to the site. As I said, a tax on site value would replace all taxation of labor, production, and exchange, so this would explicitly eliminate all tax disincentives for the creation of human and physical capital. Indeed, that's pretty much the whole point of site value taxation.
PB) I prefer a head tax (flat fee for each citizen) whenever possible, for major infrastructure projects - such as a levee, to prevent flooding (PBA capitation tax is certainly better than a tax on labor/production/exchange, but it is not an optimally fair or efficient way to finance projects (like levees) whose costs get capitalized into increased site values near the project. A site value tax would collect the increased ground rent created by the project, whose costs might not be proportional to the number of people living in the affected area.