My domain : CreditCardPaymentGateways dot in even after 6 months is not ranking well. Could it be due to length of it?
Also I've blocked backlinks to some directories( without nofollow) using robots.txt.
I suspect either is playing spoilsport in good rankings of my site, which I've optimized well and has least competition.
I've never heard of the domain name length affecting SEO.
Google doesn't appear to show any inbound links to your site (via Google Search) - that probably won't be helping matters.
About your specific website
Assuming you are talking about the ".com" website: I am sorry to say, but about everything is wrong with your website when considering the "rules" you have to follow to gain proper SERP results. A good starting point would be to start reading the webmaster guidelines from Google: http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=35769
About the question you posted
The search engines would are not necessarily "punishing" websites that have "long" domain names. However, from a users perspective you would want to read a domain name that is easy to read and one that reflects the contents of the website it represents. This DOES NOT mean summing up a string of keywords and place ".com" at the end of it. It has no added value and might even be seen as a "spammy" activity.
Domain length should not impact your chances of ranking well. However taking a look at:
Your site it would appear to be a holding page with low value content this probably means you don't get much traffic. Plus your domain name is made up of a number of keyword phrases so I assume you were wanting to rank for exact match searches.
I would look to improve content and do some link building.
