There is an issue. The quality of a work depends on the extent of motivation to improve that work, and motivation is primarily derived through competition. It is simple to see that the most contested games are best in quality. Using words like "art" and "entertainment" are difficult to place in competition because these are subjective judgments.
Recordings need to be composed to objective ends, and their latent entertainment value follows from their quality. If the recording is not entertaining, then it may be said that it either needs more appropriate objectives for entertainment value or that it is not a good game to compete over.
Entertainment is a construct: it is intangible and imaginary; as such it is impossible to measure objectively. Replacing the word entertainment with another construct will not change anything--the word "quality" being an example. Unless measuring them only in terms of speed, what makes a decent run remains similar to what makes good music: there can be guidelines, but very few rules.
measuring them in terms of speed, terms of in-game decisions (challenge level for example), or game recorded achievement to name a few. there's other goals that can be measured objectively. it's those that drive its quality.
edit:
entertainment (or any other construct) as an indirect result, not as a meaning to initiate. a recording to be judged by objective measure is one that has meaning to its author and is one that is capable to compete over.
There can't be such thing as objective judgement, anyway. Any kind of judgement involve subjective measuring, and sometimes it may help.
Imagine a situation when one TAS is 30 minutes long, but it's full of action and don't have dull moments at all — even pauses are filled with some different attempts at entertaining the audience. Another TAS for the same game is 30 seconds faster, done frame perfectly, but is boring to watch because all the action is utterly predictable, every in-game pause is a pain to watch, etc. Objectively, the former is slower and the latter is faster. And subjectively? I guess it'll be quite the opposite.
What method of judgement should be applied in this case? (BTW, even this choice will be purely subjective.)
I agree that such words as "art" and "entertainment" are pure subjective things: each of us, being influenced by various experiences and sources of information, have his own definition of art and entertainment. I guess it may depend on the judging audience, the game itself, the author of the run, the chosen route/strategy/whatever, etc. So there can't be any trustworth measures of entertainment in the TAS itself — it's up to us — the individuals — to decide, what's entertaining and what's not. Any kind of polls may be misleading and nonrepresentative in this case.
But, with speed being the primary objective for every TAS, what to do with the situation described above? Is it entertainment or is it perfectionism? (Yes I know there should be both, but not all cases allow that. Besides, there may be no other choice but to obsolete one of the runs — especially if all other TAS characteristics are identical.)
Edit: added emphasis.