There are a lot of positive elements to this design and I think you're on the right track. While I generally like the direction there are many things that don't look right, are misleading and don't quite work. Below I'll try and outline a few of those things:
This first thing that really hits me here is that I don't really know what you're pitching. After reading your paragraph description, it's very clear, but most users aren't going to get that far. The point of the site needs to be blatantly obvious from the start. When I first saw it I thought it could I thought it was just a worpress theme. You're heading is very clear after you know what it is, but not until you read the paragraph. Maybe set up a little info graphic, instead of the heading, explaining the steps:
Pick out a theme that you like
Take it for a test ride
Buy it (if you like it)
All of these items would be, of course, accompanied by a picture. I think this would go a long way to explaining the site.
Much of your design is spot on, but then there are some nagging little areas that really take it down for me. Design is all in the details. The details show that you spent time, you care and you know what you're doing. From alignment, to spacing, to placement, make sure everything is just right. Take a look at my notes for further information.
Last but not least is Usability. This is what will make or break your site. Sadly, I think there are some big usability faux pas here. Your login button is barely viewable, this should go near or with the navigation. You blog takes away from the purpose of the site and distracts, link maybe, entire section...no.
All in all, I think this site could really go places. Focus on the small things though. This is not a situation where you don't have to sweat the small stuff. Take time and perfect this piece. I hope this helps you in the long run and I look forward to seeing what you come up with.
This hosting link seems awkwardly placed, as it doesn't really relate to the core purpose of the website. I would turn the hosting link into an ad, which you could place at the bottom. Alternatively, I would re-title to "need hosting?" That would make it more obvious that it is an extra element rather than a core feature.
There's a bit of redundant repetition here: "Try before you buy" which is immediately followed by "Try Premium themes". I would amalgamate them into one. It would make the design less cluttered and increase impact.
This text is still a little awkward. Part of the problem is that it's hard to pick out the name of the website. (Although you fix that easily by putting it in bold). I would re-write it as follows:
Welcome to Wordpress Themes Preview! We let you test out premium Word Press themes, including all their control panels and features, for absolutely free!
perfect theme for your site with our Free, No Obligation Service!
Have you considered making a logo?
Even a simple logo would make it clear that this website is call "Wordpress Theme Preview". At the moment, it's hard to work out the name of the site (As "WordPress Theme Preveiw" sounds more like a technical function than a business name).
This area doesn't work as well as it could. I think most people would expect the opening section of a website to have a lead image or a featured theme or some other element to visually convey the purpose of the website. Visually elements convey information instantly, where as textual elements require the user to spend time reading them.
The green background in particular doesn't seem to relate to anything and the whole area seems like an afterthought. It's not a major problem, but it seems a shame to waste the potential of this space.
Typically login buttons are expected on the top right of a site. Try moving it there, this is a usability best practice.
If you have to move items that are usability best practices, you better have a damn good reason.