92% of all users that apply to Google Adsense are rejected. The folks over at Adsense seem to look for a few things in a blog/site worthy of displaying their ads, including the content, design, and the age of the site.
If you have little content on your site, it will be rejected. A good rule is to have about 20 pages of good content before applying. Also try to get some incoming links and some visitors participating on your site before applying.
If your site is poorly designed, or doesn’t look professional, you will be rejected. Spend a few hours to modify a Wordpress theme a little bit to make it your own.
Age may be a factor as well. I would suggest letting the site age a bit before applying. I typically let my sites build popularity for a few months before placing ads on them. Think of it this way: even if you got accepted on the first day your site was launched, you probably don’t have too many visitors. In fact, you’ll probably make all of 5 dollars during your first 3 months. Is it worth the 5 dollars to wait and do it right? Of course it is!
Also be sure to remove any other type of advertising you are doing on the site before applying for Adsense. You are not allowed to have contextual advertising side-by-side with Google Adsense. You can add your non-contextual-based ads back in after you’ve been approved if you’d like.
Finally, know that after you get approved for one site with Adsense, you can place your ads on any sites. So if you are working with multiple sites, just choose the best, most mature and professional-looking site you have to submit.