Regarding Carnac (or probably more so Stonehenge, as posts above indicate Carnac isn't quite as closed-off as I previously perceived), the main thing that will still kill it for me is the fact that there is a need to apply to see it and that the visit will be under the scrutiny of the locals/EH. It doesn't make for much of an atmosphere in which to really appreciate the place, at least not to my mind. I think I like solitude and unfenced open space too much to really enjoy that. It's one of the reasons I much prefer Wales visits to England, because the constant barrage of "keep off the grass/no right of way/private" etc signs lessens the enjoyment - just reading the signs is dispiriting and rather soul-destroying. I experienced this feeling on Saturday while walking (along pavements) through the suburbs of Bourton on the Water. Everything was covered in notices and signs.

On the "monkey see, monkey do" point, I know what you mean but still can't really see that climbing on top of a trashed mountain top cairn is going to do any harm to it, unless you start messing about with the stones. So if they copy me doing that, I'll live with it. It won't cause any more harm to the ancient monument itself than climbing a set of stone stairs in a medieval castle will, or walking round the ramparts of a hillfort or banks of a henge (in fact, probably considerably less than those last two).

Oddly, I remember getting very annoyed (in a silently disapproving way, inevitably) when I saw someone climbing on top of a trig pillar on one Welsh hilltop. Although only half a century old, these crumbling concrete pillars are much more vulnerable to damage than the granite blocks of a cairn. I can't really explain why I was so irritated by it, it just seemed "wrong".