Most CMS have some sort of restrictions on what HTML tags the user can input, including some that cost hundreds of thousands of dollars. The reason being that the end user of the product probably doesn’t understand the nuances of (x)HTML, and therefore may break something if you give them too much power. Of course, most CMS have a much easier way of turning this off too!
I must admit that I am a big WordPress advocate, but this stems not from the inner workings of the product but the sheer ease of use that offers the end user. Of all the systems I put in front of my clients, WordPress is the one that continues to impress.
The idea is that you (the designer) provide the end user with a custom template or a plugin that provides all the functionality they need. This leaves the content area of the post/page for simple content that the end user can manage themselves. I think this is a much more elegant solution than "don’t edit this page it might break", but that’s just a personal preference.
So anyway, there’s a couple of filters that you can turn off to make WordPress "get out of the way". They are wpautop and wptexturize. There is a handy plugin that can help you with this process (and more). It’s called Text Control. You should definitely check it out as it might make your life a bit easier.
Hope your future dealings with WordPress are happier :D