I'm happy to be a part of this wonderful community here and request help analysing my situation and would appreciate suggestions to fix the issues I'm facing.
First, The Website Setup -
We've a WordPress powered technology blog on main domain - http://www.crazyengineers.com & community forum on /community/ In the last 60 days, I've made following changes to the site that might have gone against us.
1. I added a sidebar to all the pages of our community forums to offer more useful information to the users, like recent threads, our latest exclusive interviews etc.
2. I added a descriptive welcome notice on all the pages of community at the top.
3. I added two ad-spots above the fold, didn't hurt user experience (IMHO).
4. I removed a LARGE number of unwanted tags from our WordPress blog that offered no value to visitors; resulting into Google Webmaster Tools reporting a large number of 404 - Page Not Found errors.
Point To Note: Since September 4, I've been noticing significant drop in our traffic to the 'community' forums. The traffic is down almost 40-50%. Corrective Measures I've Taken -
1. Made sure that all the removed tags show proper 404 HTTP status. I added a search box to the 404 pages so that users have opportunity to find what they're looking for.
2. Removed the two ad-spots and placed only one 728x90 leaderboard above the fold.
3. We never engaged in any black-hat SEO techniques, link-building or content-copying. I double checked whether there are any suspicious activities reported in GWT but haven't spotted any.
I'd really appreciate analysis of what might have really gone wrong and the direction in which I should proceed to recover our traffic.