Well, a critique is just that, a critical commentary on the image. Compliments aren't critical.
That said, I have always found that the best way to get a constructive point across is the tried and true compliment sandwich.
Nice thing, critical thing, nice thing. It all has to be true, though. Don't say something nice just to fill the air with noise, and don't critique without reason or invitation, either.
"Great color and contrast, I find the tilted horizon a little distracting, but the line of the seafoam is a great focal point."