5...h6 also seriously weakens the h5-e8 diagonal. This is not just a theoretical problem. After 6.Bxf7+ Kxf7 7.Ne5+, Black cannot retreat his king to e8 because White has mate in two because of the weakness on that diagonal.
So yes, h6 was that bad. And the explanation for why it was that bad is right in front of you in the analysis that you posted in #1. White had devastating tactics available on move 6 and move 7. You could gain a great deal by studying those positions, because those are very thematic tactics that occur frequently.
Lots going on here!
Firstly, to @laskersnephew above, I just wanted to understand why that line was SO bad, because I have seen Queen blunders that didn't result in a 8 point swing, and it's not immediately obvious to someone of my level how to make it a game-winning position after move 8. You've effectively said the answer was in front of me, but in fairness, if that had been adequate to sate my wish to understand the reason why, I wouldn't have posted anything at all.... It is correct that studying positions is useful, and that's why I even study my opponents' errors and have moved from 580 to 1050 in 5 odd weeks, and hence why I asked the question
@pfren and @RAU4ever thank you for your insights, a few things I've noticed include.
a) It seems really easy for people to get very divisive very quickly here, I imagine I was explaining to a child some fundamental of physics, and Einstein's in the room and says I am an idiot who understands nothing of physics and just having a Bsc in it should totally shut up and not talk to children about physics..... I guess we have to accept that people will have different opinions based on lots of factors, not the least what level they themselves are at.
b) It seems really easy to get side-tracked I didn't even mention the a6 at the start (although to be fair I did in post #8 in passing), yet a lot of conversation got very focused around that element.
c) It's quite hard to keep a discussion at a level that is appropriate for the poster. I get that each individual will vary, however I do like that comment by @RAU4ever that we learn to count before we introduce -1^1/2, and a lot of the conversation here is going over my head right now. I actually don't really consider myself a "beginner" now, but definitely a novice There seems to be quite a lot of looking down on anyone < 1400 as some sort of idiot by a certain stripe of person - which doesn't really help. But explaining only the concepts relevant to me at my level would certainly be helpful.
I think my final "answer to myself" would be something like...
The move allows a sacrifice combination by white that at best mates in 4 and at worst drives the king out into the middle of the board, and yes, that is worth an evaluation swing of 8 points.
This is borne out by the next move 6. d4 swinging the eval bar back to +1. The lack of development is a factor in white having an overall better position regardless, but most of the swing to +8 was due to that possible combo move.
Many thanks to all who've written genuinely helpful, or even just interesting comments, I think the above is the "answer" to the simplish question I wrote in this "beginner" forum
Now to get back to learning the next steps for me at my level in my time :wave
Thanks again!
Nah, nobody should feel bad for being <1 400, I was there last year, nothing wrong with that.
Generally the reason why many of us state that in the beginning (beginning is the colloquial term as in mostly everything less than 1 200 or so) you should really try to develop the pieces as fast as you can, because of a game like this. There are many somewhat lower rated people who feel the need to play these a6 and h6 type of moves.
You do not really need to remember something like: "it fails because of this sacrifice and then there is this precise sequence of moves."
You should remember the principles. What you should learn is to just see the position when the evaluation shifted (it is on move 6) and see that white has 3 pieces out, black has none.
Sometimes you can get away with that, but in general 3 pieces out vs 0 means that the side with 3 pieces out is much better (in some cases like in this case even winning). Of course, here it is even worse because there is a concrete tactical shot, but the concept of quick development is something that will serve you well at almost all times.
By the way, this pattern with queen check on h5 is useful to remember as the variation of that tactic can happen from time to time. For instance it can happen when you move your f pawn too early (which is another thing novice players often misuse), and in that case it is even easier to spot because no sacrifice is even needed.
As for saying that you do not see an immediate win, you are correct. If black doesn't go to e8, there is no immediate checkmate, but visually speaking if black plays correctly to let's say e6 with his king, black has all the pieces at the back rank and his king on e6.
White is a piece down, but that position can't be good for black because king at the center of the board will either be checkmated or it will result in a big loss of material. Black has sacrificed a piece for a pawn, lead in development and a very unsafe black's king. It seems that white has more than enough for the sacrifice.
Of course, white can mess up the position as well, but that is up to white. 
"
5...h6 also seriously weakens the h5-e8 diagonal. This is not just a theoretical problem. After 6.Bxf7+ Kxf7 7.Ne5+, Black cannot retreat his king to e8 because White has mate in two because of the weakness on that diagonal.
So yes, h6 was that bad. And the explanation for why it was that bad is right in front of you in the analysis that you posted in #1. White had devastating tactics available on move 6 and move 7. You could gain a great deal by studying those positions, because those are very thematic tactics that occur frequently.
Lots going on here!
Firstly, to @laskersnephew above, I just wanted to understand why that line was SO bad, because I have seen Queen blunders that didn't result in a 8 point swing, and it's not immediately obvious to someone of my level how to make it a game-winning position after move 8. You've effectively said the answer was in front of me, but in fairness, if that had been adequate to sate my wish to understand the reason why, I wouldn't have posted anything at all.... It is correct that studying positions is useful, and that's why I even study my opponents' errors and have moved from 580 to 1050 in 5 odd weeks, and hence why I asked the question
@pfren and @RAU4ever thank you for your insights, a few things I've noticed include.
a) It seems really easy for people to get very divisive very quickly here, I imagine I was explaining to a child some fundamental of physics, and Einstein's in the room and says I am an idiot who understands nothing of physics and just having a Bsc in it should totally shut up and not talk to children about physics..... I guess we have to accept that people will have different opinions based on lots of factors, not the least what level they themselves are at.
b) It seems really easy to get side-tracked I didn't even mention the a6 at the start (although to be fair I did in post #8 in passing), yet a lot of conversation got very focused around that element.
c) It's quite hard to keep a discussion at a level that is appropriate for the poster. I get that each individual will vary, however I do like that comment by @RAU4ever that we learn to count before we introduce -1^1/2, and a lot of the conversation here is going over my head right now. I actually don't really consider myself a "beginner" now, but definitely a novice
There seems to be quite a lot of looking down on anyone < 1400 as some sort of idiot by a certain stripe of person - which doesn't really help. But explaining only the concepts relevant to me at my level would certainly be helpful.
I think my final "answer to myself" would be something like...
The move allows a sacrifice combination by white that at best mates in 4 and at worst drives the king out into the middle of the board, and yes, that is worth an evaluation swing of 8 points.
This is borne out by the next move 6. d4 swinging the eval bar back to +1. The lack of development is a factor in white having an overall better position regardless, but most of the swing to +8 was due to that possible combo move.
Many thanks to all who've written genuinely helpful, or even just interesting comments, I think the above is the "answer" to the simplish question I wrote in this "beginner" forum
Now to get back to learning the next steps for me at my level in my time :wave
Thanks again!