Well then maybe you shouldn't review it.

So, I hate it when I'm on Amazon looking for some books to read and one of the reviews says "I DIDN'T GET IT--DON'T BUY IT, IT'S A WASTE OF MONEY" (mind you these are the books that cost like 99cents so I'm not sure what money they're talking about).
If you don't understand something, your opinion doesn't matter. I want to read a review by someone who understands the book enough to write an educated response. If you really think $.99 is a waste of money....well then, you really shouldn't be buying books. You should be saving your money. Even if you don't like it, IT'S 99 CENTS! It isn't going to kill your bank account.
And it's even worse when someone is like "I only made it through the first chapter." If you don't read the whole book, you shouldn't be telling people that it's a bad book. It's one thing to say "I couldn't get into it." It's another thing to say "I only read the first chapter and I think that this book is stupid and you would be stupid to buy it."

So reviews should be written by people who:
-Read the ENTIRE book.
-Understood the book.
-Don't feel like it's the end of the world to spend $.99 on a book.
-Respect that some people will think it's a good book, so therefore you shouldn't bash it. You can still give your opinion, but in a respectable manner.