I agree with Le Fanu. But on one point, I differ. I do like comments on card stock and size. I have small hands and I don't fare well with large decks. I also don't care whether the art work is huge in size, so there are two reasons why I prefer small decks. I have a terrible time shuffling thick card stock and if a deck is both thick and large---well, forget it. I might as well pour money down a rat-hole if it's one I've bought. I won't be able to use it. Also, I riffle my decks, so I don't like having to tediously make little piles and then put them back together or any other shuffle other than the riffle. The riffle is part of my "process", I think.
Aside from that, I pretty much agree with Le Fanu, especially the part about having spent some quality time working with the deck. I think it's good to say, "I was blown away by the beauty of the deck when I first unwrapped it" but then to go on to truthfully express your experience with it if it wasn't very positive. There are folks here who read a lot like I do so their experience with a deck is likely to be similar to what mine would be many times. Not always, of course, but it does give a pretty good indicator of how I'd react to it. I wouldn't like it if my friend gave a rave review to a deck he or she really didn't feel that way about.
I see nothing wrong with making comments here on AT that accurately reflect your feelings about a deck even if they're not positive, so long as you don't rip it to shreds and get nasty about the whole thing. I don't want to know only the positive things. I want an honest review but with compassion for the artist.