Play test is a hard at times at best...

You are right on several different accounts. Something I find hard is when someone plays a game and says I didn't like it because of the theme. Change the theme and I would have liked it more. Because then you are left wondering, how good of a game could it be if what was making or beaking it was the theme? That kind of feedback is generally the scariest in my book.

Distilling the useful comments out of a playtest session is a challenge, I agree. Usually, a lot of comments are about the balance of the game. This unit is too expensive, that ability is too strong, the scoring is unbalanced. Those type of things are typically easy to recognize and easy to fix. I call that tweaking the numbers.

The hard part is recognizing when people think the game just isn't fun, and the comments they have about the balance of the game is just masking the fact that they simply didn't enjoy it much. I have prototypes of games where I spent a lot of time tweaking the numbers, but the reception was still lackluster despite the game finally being balanced. Then you have to find out why people don't enjoy it, and then you come to the crux of game design, how to put the fun factor into a game. There's no magic formula for that, of course, otherwise everyone could make the next Puerto Rico.

I usually don't get much comments about the theme of the game. Perhaps because my pool of playtesters is used to playign eurogames where the theme takes a backseat. But I do recognize that what people say after a playtest isn't usually what they actually mean. Like I said, understanding what they actually mean is the hardest part. The best indication whether a game went over well is when playtesters start a postgame discussion about the strategy of the game, rather than about the balance, theme, mechanics of the game.