To Review or not to Review

I have been spending some of the little free time that I have reading excerpts from eBooks on Smashwords which has been a hell of a lot of fun. I came across quite a few authors who have great ideas but are not always well executed and some work that is so good I cant fathom why the author hasn't had a traditional publisher throwing a contract at them.
I wonder how long it will take before it is common place for agents to spend some time trawling places like Smashwords looking for talent. I assume from what I hear that there are slush piles everest high at most major publishers and the idea is that they read these manuscripts time permitting. I guess I understand the idea of wanting to read all of something but surely if you are going to decide to buy it or not you can tell after the first few pages whether the author has got their craft right. I really do think that just because something is written it does not mean it has to be read. Its only if it is good that someone will take that time. So why don't publishers give manuscripts a quick read and if the craft needs work send the manuscript back saying so. If the writer hasn't got the craft right and keeps throwing the reader out of the story then just say so. If they keep getting it wrong with the 'show don't tell' rule, its obvious on the first couple of pages.
So with this in mind I am considering doing reviews on this basis. I don't have time to read everything in depth and I'm afraid that even if the story is good, crap writing will mean I move on to the next piece. The thing is I don't want to hurt anyones feelings and just because I might think a particular piece of writing is just not good enough it doesn't mean that future pieces wont be. Also being a fledgling writer myself I want to know what people think of my writing. So I am thinking that if I do decide to do quickie reviews of work on Smashwords and the like I had better put myself up for criticism as well. In truth the feedback that I have gotten so far from friends has helped me improve my writing. I like to know what it is I'm doing wrong. Learning from mistakes is the only way to get better.
At the end of the day we are talking opinions not facts.
[UPDATE: I decided that if I cant say something nice then its best to say nothing at all. So for pieces that I didn't like at all I am just not going to mention them, well at least when I have only read a few pages.]