I think the answer to this question, is at least twofold IMHO.
And highly depends on the point of view (as said before in this thread).
When target group is -younger- children; a good set should be fairly easy to build, has a high playability factor and should be as a realistic as possible.
For the more enthousiastic technic buyer, other factors are more important. You have basically two or maybe three kinds of buyers.
One group finds technical challenge and versatality (what else can be made/built with it?) most important.
While others like realism more.
And maybe a small group which wants best of both and are willing to make consessions in that.
And as for TLG itself; always most profitable and price are taken into account. A highly overpriced set will be bought only by a small amount of people. The more people, or different target groups are attrackes the more buyers it will have.
Re-use of parts, looking best for as little parts as possible, etc.
For me:
I am willing to pay 100+ euro for a set which has;
* A lot of -usefull- parts
* A lot of different parts, only bricks or only one kind of gears are no use, off course
* A challenge in building and a technical challenge (taken). So dificulty basically
* learning capabilties, when building the set one learns -new?- ways of taken technical burdens.
Nice would be;
realism and playability. But these are not as important. As I am not likely to go around playing several days (unlike children).
The 8043 and 8285 sets I own, are good examples.
The Tow Truck looks very nice but is not very challenging, technically. But it is huge in parts
And I got it second hand for a reasonable price
Whereas the Excavator has complexity, realism, good parts (& 4 motors + remote controls!) and is playable as well.
The 8043 is by far the best set around at the moment in my opinion.
(too bad it has the problems with the LA 's