After merging two sites to a third, using proper 301 redirects and transferring the domains in Google webmaster tools, overall traffic dropped by ~50%. One of the original sites was getting a plenty of organic traffic and had lots of backlinks. I have a feeling there are a few SEO mistakes I made during the merge.

This is a two-part question.

1. If one is diligent in fixing SEO mistakes and patient, is it possible to regain most of a transferred / merged site's lost traffic?

2. What are a few of the more critical SEO mistakes that can cause a site to lose traffic after a redesign, transfer or merge?

I have a feeling in my case, the traffic loss has to do with the merged site having ~700 URLs (due to a forum) as opposed to the old site having ~40. However, I want to know the most important items I should be checking, in case noindexing the ~650 forum URLs doesn't work.

After merging two sites to a third, using proper 301 redirects and transferring the domains in Google webmaster tools, overall traffic dropped by ~50%. One of the original sites was getting a plenty of organic traffic and had lots of backlinks. I have a feeling there are a few SEO mistakes I made during the merge.

This is two part question:

1. If one is diligent in fixing SEO mistakes and patient, is it possible to regain most of a transferred / merged site's lost traffic?

2. In this particular case, the traffic loss may have to do with the merged site having ~700 URLs (due to a forum) as opposed to the old site having ~40. Is there a high probability that this is the problem causing the loss in traffic? If not, what is the next thing I should check?