1. you said plain.. but then everything is a big mess? .. how does this goes together?
Design wise, your site is plain, it's boring. As for a big mess, due to lack of design all the elements on the page run into each other. There is no definition.
2. it's in php?
And? PHP is a server side language that after it has processed outputs (X)HTML (most commonly, but it can also output numerous other things) to the browser. I wasn't checking your PHP, I cannot do that unless you show me, but I was checking the HTML that the PHP creates, and this is not valid code.
3. contrary, i have good search engine hits from google, yahoo but lesser from msn
No one except the search engines really know how it all works, but it looks to me like your spamming it and if someone from Google were to look at your site they might think the same thing.
Joomla is a CMS, like many many others. But, unlike many other good CMS's, Joomla uses outdated and unnecessary code, and doesn't validate. And what exactly do I need to check myself for?
I'm sorry if my post wasn't what you expected, but you asked what I thought.