Well, you address a very interesting and important point in general here, even if the particular question with which you do so is a bit wrongly premised. But let's try to look at the whole thing with all its intricacies. First of all,
The broader cultural backdrop of the site's community
The matter of identification questions is a very controversial topic on the site. There are some users who like them and many users who dislike them for the steady stream of low-quality content they bring that is of very low long-term information value. While they bring many new users to the site, those users often won't stay active at all and some users feel that on the long term those questions will drive more avid users away by slowly turning the site into an uninteresting quiz show.
This is not to say that all those questions are to be downvoted just for the sake of downvoting but to give you a taste of the fact that while accepted as on-topic questions, those questions are still frowned upon by some users and there are various measures in place to actually control their quality and filter out the very bad one-paragraphers that just throw a scene at us and see what comes back. There is a close-reason for providing not enough detail/effort, there is a manual maintenance task for deleting old inactive ID questions and there is an automatic tag-based popup to encourage more details before asking the question.
The approach of downvoting bad questions
However, a large and important way for quality control on any SE site are votes. Good questions are to be upvoted, bad questions are to be downvoted.
If the tag is valid, then valid Q/A should not be downvoted
No, you are mixing things up here. Downvotes are not about validity, that is what close-votes are for. There can be perfectly on-topic and valid questions which are still just bad questions.
Just supposing the downvoters to be "trolls" is a very negative and defensive approach to what is a natural quality assurance process of SE. Granted, it is indeed preferable to provide some reason for the downvote and guidance for improvement, and often this is done for ID questions that need some more detail. However, not everyone refraining from doing so is a "troll" either. There is a reason downvotes do not require a comment as Mike explains in his answer.
Neither is your proposed tag wiki excerpt a reasonable idea. It is a good idea to encourage more details for the question in the tag wiki excerpt, but actually discouraging users from voting, no matter in which direction, is absolutely counter-productive to the whole SE philosophy. People should definitely vote, up and down, to show their assessment of a posts quality. And moderators don't have anything directly to do with it either. The moderators won't run around and tell everyone to "vote properly" because that is a given everyone should be aware of, whatever one's own exact definition of "properly" is. Neither will they enquire the downvote reasons on each and every downvoted post.
The problem of a repelling and unfriendly atmosphere
I would not go that far, but you are addressing a problem that we are aware of. Due to the very low quality of a large part of identification questions and the resulting downvotes on them, some users who might not be too aware of the workings of SE might get repelled from the site right from the start, which I agree is not a pleasant situation for anyone.
However, this is to be tackled at a much deeper point. We got to work more on encouraging better identification questions in the first place (and improving the tag wiki in some way could be a good start, which I just did thanks to your question), not on lowering people's quality standards. There have been some approaches into that direction and I was about to post a bigger meta discussion in this regard, which I admittedly didn't get around to doing yet, though. But seeing your question and other recent ones in that regard, it might be about time for that.
[tag:identify-this-movie]for tags. Backticks are only meant for code.