Invariably the size of the blocks is mentioned (on the order of several tons to several hundred tons), as well as the issue of moving them, positioning them, and of course the precision involved in their placement.
For those of you who aren't familiar with him, Wally Wallington is a retired construction worker in Michigan who has demonstrated that simple tools, ingenuity, and a basic knowledge of the physics involved renders these issues moot. In other words, this guy can move massive rocks, by himself.
There have been arguments that question whether these techniques would've worked on the pyramids or in other places, but those questions are irrelevant. Wally Wallington isn't suggesting that he is using the explicit techniques of our ancestors, but instead he is demonstrating that there is a means available that doesn't require advanced technology and thousands of workers. The ultimate point here isn't to examine whether Wally's methods are historically accurate, but simply that they can be done at all.
This completely destroys any assertion that ancient astronauts or mysterious powers are necessary to have built these structures. It is clear that they are well within the capabilities of the most primitive tools.
Similarly there have been critics of the Coral Castle (a modern day "mysterious" building) tripod photos, where perhaps it would be more useful to demonstrate why they couldn't have worked, instead of criticizing images that clearly show them working.
It seems obvious that there are far too many arm-chair engineers that want to propose theories and ideas of what our ancestors could or couldn't do, rather than getting their hands dirty and following Wally's lead and actually figuring it out. As with many aspects of "experimental archaeology", the purpose is not to conclusively show that a particular event occurred in a particular manner according to the experimenter (i.e. Thor Heyerdahl's voyages), but rather that they are possible with the technology of the time. This is sufficient to demonstrate that no extraordinary means or knowledge are required to lend credence to a particular method. Once it has been demonstrated that it is possible, then evidence may (or may not) be found that can focus on specific techniques that may have been employed. This becomes especially important when archaeological items or structures being investigated may have no historical records. In those cases, it is likely impossible to determine precisely what specific techniques may have been used, so the first order would be to assess plausibility.
So the next time someone talks about "Forgotten Technologies" and how no one knows how our ancestors could've built these structures, ask youself, how much effort has been expended in following up on these modern day explanations and examples. That will be the best indicator of how seriously these questions should be considered.
Whether aliens ever visited this planet in the past may be a question open to speculation, but most assuredly we can be confident that, if we were visted, they didn't come here with the intent to build pyramids.
"The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt." -- Bertrand Russell
* There is a similar structure in Florida, called Coral Castle. While it is often associated with mysterious processes and questions regarding how it was built (I suspect that is more PR and marketing), the reality is that the owner/builder (Edward Leedskalnin) also utilized his knowledge of leverage and gravity to perform the construction. Despite claims to the contrary, here is a picture of the builder working at lifting one of the stones.