Of course everyone is entitled to mark them as they see fit, but I'm just trying to put across my ideas.
Whilst agreeing totally with that comment I might as well put forward "my 2 bits".
I have determined that an "average" WHS deserves 2.5, initially assigned that to all my 767 visited sites and then considered each of them individually, to assess which are a bit "special" and deserve more points or were "under par" and should lose against the "norm" for a WHS. I could have chosen 3 as "standard" but 2.5 allows slightly more differentiation (5 points) for "above average" sites than "below" (4 points) - hopefully there should be fewer sub-standard sites than "stars". A rating of 2.5 means that the site delivers what I would expect of a WORLD Heritage Site - i.e it offers something "worthwhile" in that "company" and, to my unavoidably partly subjective, eye provides to its visitors the "O", the "U" and the "V" they would have hoped for - even if it doesn't necessarily cover a domain I am (or they are) personally greatly interested in! In short, I would recommend anyone interested in travel (i.e not necessarily a "WHS traveller") to "go see" a site at this rating level if they were in the area. As would be expected - the majority of WHS achieve at least this rating.
I have given additional ratings where I regard the site as having something "extra" for the non-expert but generally "interested"/"aware" traveller (ie. avoiding what I call the "Salamander Syndrome" by which the Smokies are the best place in the World to go to if you want to see species of Salamander - but does that justify increasing its rating from 2.5 to 5.0? I guess it could if you are a Herpetologist!!) This "extra" could well have endowed such a site with a widely appreciated "iconic status" whether for its location, scenery, megafauna, architecture, history etc which would provide most intelligent people (not just "herpetologists" etc!) with a degree of "wow factor" when they visit and would justify extra effort, cost etc in making such a visit. Finally, of course, I can't avoid incorporating my own personal interests (e.g I don't particularly like large cities but do like archaeological sites!) which will inevitably make an additional rating difference - but, hopefully by not too much!
On the other hand, sites lose ratings against my average if I feel that they are just another (and not so "good") representative within an already well represented category or if their OUV is somewhat "dubious". We all know that, despite the "theory" that objective assessment by a neutral AB is a prerequisite for inscription, WHS do get inscribed via "political" decisions!! Sites whose main OUV is very esoteric (at the "Salamander" level for instance) or virtually impossible for the average visitor to see would also get marked down - but I DO allow (without marking down) such OUV to be presented in an on site museum or replica which requires a visit to the site to obtain - you would still want to visit in order to see this "added value" and the site's general environs. In general I would NOT feel able to recommend non "WHS travellers" to go out of their way to visit such low rated sites without caveats or even (at the lowest levels) "at all"! (knowing of course that "real" WHS travellers will happily travel hundreds of kilometers to look at nothing more than a cave entrance, a flat expanse of water or a forest with a pole in it!)
The issue has been raised above, of what to do if one hasn't seen "much" of a WHS - e.g only one of its "locations". I am happy to "use" what I did see plus my background knowledge of the other locations to assign a rating. In most cases one will have seen one of the more important locations anyway (or, as in the case of Struve Arc, one which is as "good" as any other!). The same applies if e.g I have only been on 1 "tour" of a palace and thus haven't seen "all" of its rooms! Similarly for flora/fauna-related "Natural sites" where I haven't had great views of that particular significant Flora/Fauna. I have assessed what was generally on view and what one might hope/expect to see even if I personally wasn't completely "lucky" on my visit!! Similarly regarding other chance-based "visit experience" aspects ("rainy day", "received explanations from the director", "had the best view of tiger I have ever had", "the flowers weren't out at that time" etc etc). It is the site which is being rated and I have tried to discount the particularities of my visit and mood at the time!
I have also considered how to handle the issue of the value of the "journey" in rating a WHS. For many WHS travellers the excitement and difficulty of getting there increases the perceived "value" of what is there. Tips on logistics are always worthwhile but just read some of our reviews which seem primarily to be about the journey "there" and sometimes leave me asking myself "yes but what was there when you got there!!". I have decided not to assign any "value" to remoteness or danger etc etc. whilst recognising that facing these might well be a major factor in some (WHS) travellers choosing to visit some sites.
My personal %ages of visited sites by rating currently stand as follows (but will no doubt change before I record them via the to-be-delivered bulk update feature!). Not a perfect, but a reasonable, bell curve, skewed slightly towards "excellent" rather than "poor" sites but still, correctly I feel, recognising that most WHS are as "good" as each other to the "aware traveller". (I have just checked for the "Nationalism bias" issue raised above by Meltwaterfalls - my 28 visited UK WHS have an average rating of 2.62 as against 2.74 for my entire "visited" population. Some might say that even this is a bit high for UK but I don't think a charge of excessive nationalism would stick!)
5.0 - 5.1%
4.5 - 2.6%
4.0 - 6.0%
3.5 - 4.9%
3.0 - 9.6%
2.5 - 51.6%
2.0 - 17.2%
1.5 - 1.4%
1.0 - 0.8%
0.5 - 0.8%