The guidelines as they are written are not very good. It would seem that any historical marker or building cornerstone could be made virtual cache. If the marker can be used as an offset to a physical cache why not do that. You would still get people to find out about whatever it is you think is interesting.
Here's my interpretation and comments on the guidelines.
1) A cache must be a physical object that can be referenced through latitude and longitude coordinates.
Not just permanent or semi-permanent. There must be something you find - a sign, a marker, etc. Not a park or a building. It would make the virtual cache better if the object is something that you need to search for (and not an obvious historical marker).
2) A cache must be novel, of interest to other players, and have a special historic, community or geocaching quality that sets it apart from everyday subjects.
This is the "coffee table" book requirement. This should be clarified - somebody though it was interesting enough to put a plaque or marker. But should everything with a plaque or marker qualify?
3) Virtual caches are not commercial
This may eliminate some good virtual caches - There is a locationless cache to find "firsts" eg. first McDonalds, first Radio Shack, What if a realy interesting event occured here. Of course, the cache should be the plaque or sign commemorating the event - not the store where it took place. I'm thinking of a virtual cache where you would have to ready the newspaper stories on the wallpaper at Subway 
4) Virtual caches should be geographically dispersed. New postings which are within 0.1 mile of an existing cache will generally not be approved, unless the poster provides a compelling rationale.
I have a virtual I've been thinking about submitting because it is on a trail between two other caches. Several people have unnecessarily bushwacked up a steep hill to go from one to the other because they don't know about this trail. I think its ok because it is .4 miles from each of the other caches.
5) The cache has one or more questions about the location that should require going to the location and not being able to look up on the web or in the library.
Like Squirrel Nut, I was able to find the answers to the first two questions online at the Round Rock website. I think with a little more work I could have found a list of names and assumed that the names on the marker are in alphabetic order. The last question is what a lot of virtual caches end up asking for as this probably requires visit the marker.
Guideline 6) suggests that if there are no questions you could require a photo (usually with your GPSr showing) to verify you found the cache. In any event, the find must be verifiable.
7) Requires that the owner maintain the cache, that is to say, the owner will check at least once a month that the cache is still there as well as respond to inquiries about the cache.
This may also eliminate some good virtual caches that are not in easy to get to urban areas, since who wants to give up one day a month to trek out to some location in the wilderness. Rules against placing physical caches in National Parks and in wilderness areas make these prime locations for virtual caches and I assume that many get approved even though the owner may only be able to check once or twice a year. There was also another thread suggesting that vistors from out of town might want to place virtual instead of physical caches. This requirement does show that just because a cache is virtual doesn't mean that it doesn't have to be maintained.