Google representative Gary Ilsh answered blogger Jennifer Slegg's question at the State of Search conference, she asked whether low-quality pages removal from the site would improve position of the remaining pages in the search results. Ilsh assured that, despite the popular belief, there won’t be an instant effect.
Ilsh stressed that the only effect guaranteed after using noindex on site’s poor-quality pages, is traffic reduction. Instead, the gradual improvement of low-quality pages will be much more effective.
The only exception is spam pages: deleting them really makes sense, since they affect site’s search results appearance conceptually. In all other cases, it’s better to spend time and effort not deleting pages, but improving their quality.
In her blog, Slegg also added that it is worth monitoring page transitions, if there are visitors coming directly from Google that means the page meets the SERP standards and is of sufficient quality.