Be cautious when NOINDEXing topics in masses.
If you made a mistake NOINDEXing a topic that has been indexed for years, it may take a while for Google to reindex (1 to several months). What's worse is that you may end up with a lower rank. You are forcing Google to recrawl/reindex the page. Older topics will get hit the hardest.
With that said, there are some pages you can safely NOINDEX. I would start with Profile pages. They suck up your crawl rate. NOINDEX inactive accounts and maybe accounts that have 10 or less posts. You can already do this with IPS.
If you have RSS generated pages, you may want to consider removing them as well. These are just copied content from external sites. Not useful in terms of Organic Search unless they have lots of comments. Manually review any that have lots of comments.
A great way to do this is to review your Analytics > Acquisition > All Traffic > Channels > Organic Search > Landing Page data. Check how much traffic a page has been bringing in the past 7, 30, and 180 days. You may be surprised to find out that some "thin content" pages are actually bringing in traffic. Compare the numbers with other pages on your site.
IMPORTANT: Make sure you are comparing the right numbers. Organic Search is all about USERS not Pageviews.
NOTE: Most sites really don't have an issue with crawl rate.
For sites that migrated to IPS (ie. vbulletin to IPS) the biggest issue occurs with change in URLs. This affects topics, images, and profiles.
The bigger your site the longer it will take to climb back up.
Many site owners don't realize how much traffic Image Search brings in until it's too late. If I remember correctly, IPS doesn't offer any redirects for attachments/images with migration. They only offer redirects for topics/forums. However, if forum URLs lack forum ids* (which some vbulletin SEO tools offered) than this is something that IPS doesn't offer out of the box. You will need custom code for this.
Before you mass NOINDEXing, I recommend the following...
Check URLs in Analytics that may be affected. DO NOT NOINDEX BLINDLY.
If you NOINDEX 5,000 topics that generated an avg 10 USERS per week. Than you are looking at 50,000 USERS lost for the week.
Let's say your site has an avg 1.5 Pages/Session. That's equivalent to 75,000 pageviews lost for the week.
Have ads on the site? The avg is 3 ad slots per page. That's 225,000 impressions lost for the week. 1 million impressions lost for the month