Copied Images :- This is being a common things to disapprove applications by Google AdSense because if you have copied some images from Google image Search then Google will disapprove your application stating "Sites Does not Comply With AdSense Policies". So, I recommended you to add only those images which are made originally by you. Well I understand that every blogger is not a designer itself so you can post your posts without adding images. Once you are approved then if you copy images from Google image then it does not matters.
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Too much Ad networks :- This can be a big cause for getting your AdSense disapproved because if you already have too much advertising networks already on your site then there is no chance that AdSense will give even the first approval to you. So, If you want to get approved you must remove all the ad networks when applying for AdSense. So that these things do not come between to approve your AdSense account. One more thing if you have added any ad-fly website entry script or any other script then that is highly recommended to remove it before applying for Google Adsense.
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Less Number of Posts :- Well as said in the starting of post bloggers just starts their blog and try to apply for AdSense and gets disapproved and AdSense stats the problem as "Insufficient Content". So, you must have around 10 genuine written posts on your blog so that AdSense might approve you. If you have less posts then 10 then it might be difficult for you to get approved.
Domain Type and Age :- Well, If you are using blogspot blogs and don't have it older then 6 months then it proved to be difficult for you to get approved. Even with 6 month old blogspot blogs it is much harder to get an AdSense account approved. So you must buy an .com domain for your blog. With .com domains there is no need for your domain to be 6 months old.
Blog Content :- Yeah, your blog content matters more than anything else. So you must have genuine posts on your blog that are originally written by you else you won't be approved because you might have heard that Content is the king. So you must have genuine content on your blog to get approved.
Age Verification :- You should be above 18 years to apply for Google AdSense. So before applying for Google AdSense make sure that you are shown as 18 years or older in your Google+ profile else you won't get approved by Google AdSense. If you are not 18 years old and wants to get an AdSense then you must have the payee name of your Father or any body else that totally depends on you.
Address not Formatted Correctly :- If you have not filled your address correctly while applying for Google AdSense then do not think that you will get approved because AdSense also takes the address of the users because the payment of your AdSense account will be send to your address that you fill. So fill out your address correctly when applying for AdSense.
Traffic :- Traffic plays an major role for you to get approved you must have around 50 page-views/day to get approved. Well sometime AdSense also approves the sites with less traffic but only sometime so be on the safer side and have some good traffic before applying. This will increase the chances of getting approved.