Download XENU - http://home.snafu.de/tilman/xenulink.html - run a crawl of your site and right click the properties of any page to see all internal links linking to that page.

404's are a natural part of websites, Google understands that. As long as you don't have links to pages on your site that are 404'ing you're fine. So basically, just make sure your website is not the source of your 404's.

No problem, don't worry about that. Just try to have the H1 with the main keyword phrase and H2 with the secondary if possible. Someone ask Cutts this question a while back and he said Google understands.

Same here - I will gladly join

I will be there, and looking very forward to meeting everyone and attending MozCon! See you all soon! - Irving

Duplicate content is not a penalty, it's a filter. Deindexing will ensure that they never rank, leave them indexed and they have a chance of ranking, worst case scenario is they don't rank well because of it.

I would refrain from using tag pages. Google does not like thin content pages and tag pages are typically just a set of links to other posts. Also, I have seen too many sites with too many unintended tags.  Category pages give you a lot more flexibility in terms of adding unique content and making them into pages worthy of ranking. Plus, category pages are in a controlled environment and there will not be any categories being accidentally formed.

"Technically speaking with www is a different URL that the one without www so you link building efforts would be split across two domain which would not be cool."

The non www should 301 to the www version, so it's not two sites, it's one. If it's two separate sites indexed that needs to be fixed asap.

But if you are www then your links should be www as well so that no page juice is lost because of the redirection.

The general consensus of my SEO peers is that approximate 15% of link juice is lost in the redirect and I second what Richard has recommended.