Out of the box WP does not support:
- Custom page titles (can be done with plugins)
- Meta description & keywords
- Permalinks like /category/ and /category/post/ (can be done with plugin but u have to modify plugin for best result)
- Any advanced stuff
- Permalinks are not very flexible. Can be configured with plugins but having permalinks like /postname/ affects performance because WP is designed by drunk kindergarten kids.
- Also, by default tag pages and search result pages are indexable which is VERY BAD! Again this can be changes using plugins like HeadSpace 2 (which btw is better than Platinum SEO and other SEO-related plugins).
Leaving aside that, 99% of the WP themes have bad markup. They have the blog title as H1 tag on every page which affects SEO quite a lot, then all panels in the sidebars have H2 or H3 titles though optimally would be to have no semantic meaning so the H2 and H3 inside the post have more weight. Basically this lowers on-page relevance of your keywords by diluting it with words as "categories", "clouds", etc. I think Google wa ssmart enough to make an exception for WP in its algorithm and not take these into consideration, so maybe it does not dilute your relevance after all.
Going into WP architecture you will soon discover it is VERY VERY SLOW. You may think it is not, but trust me. I am a PHP web developer with over 10 years experience and WP's database is poorly designed and not properly optimized and the PHP code itself is one of the worst architected and implemented applications I have ever seen. It takes about the same time to WP to render a page as it takes to Drupal, yet Drupal is an A LOT more flexible and does A LOT more processing (but is not as easy to use).
Leaving WP itself aside, many WP plugins are built by other drunk kindergarten kids. Again performance suffers and sometimes security too. For example if you have 5 plugins and all have to inject CSS, they will inject markup to load 5 different CSS files. That increases the page load time. Same thing with JavaScript.
Yes, WP is easy to use. Yes sites based on WP do rank. That does not mean Google loves WP or that WP sites perform better than others. What you think about WP is irrelevant. If you like something it doesn't mean it is the best or better than something else. Only that you like it. Show me a WP site and I will show you between 5 to 20+ things that can be improved in the on-site SEO. Also, if you only had experience with WP but no other CMS platforms and/or custom-built sites and static HTML sites, you don't really have foundation for your opinion when you say WP is great.
And I am not even going into details about why Google might not like a blog for certain keywords. A blog is a site but
a site is not just a blog. Imagine you are cNet, Amazon or some other huge site. You think you can deliver best user experience using a blog? No.