Protip: It's not considered constructive criticism if you aren't being constructive. The entire point of concrit is to bring up intelligent, well-thought-out arguments using specific details to illustrate your objective opinion. That is to say, you're not there to insult someone's work, nor are you there just to tell them what you thought. You're there to tell them specifically what worked and didn't and how they might improve so that they can learn and grow from what you have to say. If you just tell them that their story is common and bland but never bother to point out how or what they can do to stop writing in a way that's common and bland, they're just going to keep on writing common and bland literature because you conveniently forgot to specify what it is that made their work that way. It's pretty much exactly the same as me just saying "no" to you without really giving you much in the way of context about it.
In short, read this, and if you really must tell someone that their work is common, at least bother to go in-depth about it. We have quite enough of those kinds of reviewers as it is, if you don't mind, and to be completely blunt by this point, those kinds of reviewers frequently don't realize that just leaving it at the equivalent of "you suck but I won't tell you why" actually serves to discourage authors more than help them. Any sort of one-liner reviews (which it really is if you don't want to go into any sort of detail -- even just by pointing out single instances in a chapter or whatnot to show you even bothered to read what that author had to say) tends to do that.
Or to be even shorter, this entire VM is an example of constructive criticism. Notice how I'm telling you what areas you need to improve, why you need improvement, and how to improve them? Yep. That's what's meant by constructive criticism.
