Re: What are these tables?
How many servers wordpress.com is running on?
My experience with wordpress is NOT good at all. I made a website with wordpress, only 3 or 4 plugins installed (all plugins checked and optimized), 600 categories made, and so far 700 posts are there.
A single post page showing more than 35 SQL queries and on an average it takes from 3 seconds to 6 seconds to generate a Single post page. I optimized database, setup PHP eAccelerator and it is hosted on VPS.
With this much server resource, if 600 categories and 700 posts are taking making it that slow, then I can not imagine to have millions of posts on wordpress. I can not afford to have 1000 servers for one million post.
If you google a bit, you will see how many people complain about wordpress for being SLOW. Looks like there is no limit for SQL queries in wordpress and if just to receive a category hierarchy one needs to run 35 queries, then there is some serious issue with the optimization.
I am not a GURU or Genius, I respect wordpress developers from the bottom of my heart BUT I think there should be some thought about making it fast. Every time with the latest release (of wordpress) we see Lots and lots of AJAX and CSS improvements and it is making it much more slower than ever. The write post pages, you just try it on dial up or some other slow connection and you will know what i mean.
For the beginning it feels like Haven to get a site with lots of controls and free updates but as soon as you start growing, you realize that there is something seriously Wrong with your system. All the money you earn from advertising will go to the hosting companies.
This is just my experience and I am not arguing with anyone.