I am, when asked, a brutal beta reader. I have what I like to call "An eye for imperfection". If asked I will tear your manuscript a new one. I will poke holes in your plot. Pester you with failures of logic. Rake your dialogue across the coals. Call you to the mat on character inconsistencies.
However this laser perception fails- sometimes more and sometimes when faced with my own work. That's not to say that my rewrites are completely unhelpful, but they lack the insight which I can level at someone else's. This is the same principle as the shoemaker's children have no shoes or the matchmaker is always single. In some instances it's just easier to do for others.
There is a lot of advice on brutal self editing, everything from putting your manuscript in a drawer for a month to reading it aloud. The reading it aloud option is a very good one, BUT even that sage advice only goes o far.
When the time comes, after as many edits as I can stand I will be sending out Tea Times Three to beta readers far and wide and I only hope they skewer it to the wall, rip it apart, and help me make it better.