Barriers is quite abstract so I define three kinds of barriers:
- Login barrier
- Usage barrier
- Payment barrier
Login barrier
Many web applications have the need for a login. Generating a new user account for each new web application was trendy 2005, but users didn't like the approach too much. One can leverage that login process by allowing people to sign up using their Facebook, Google or xyz accounts using eg OAuth[1]. Takes the pain out of the process. But still - there are at least 2 steps to perform to get logged into something. That might be too much.
Even better: Allow people to use your application without even the need for a login [2]. No barrier. Immediate feedback for prospective customers. Only generate a user account when people are already hooked up. The will to register will be much higher. Possible downside: Collecting email addresses via forced logins is worth a lot in terms of marketing. If people can try and leave your app without the need to provide any credentials you will lose contact information.
Usage barrier
To me the most important thing is the first impression something makes [3]. If it's too complicated people will move on (unless you got some really good arguments). So removing complexity is king. Regarding usability much has been said in the past. Design your apps for users. Constantly improve your user interfaces. Listen to your users - they will tell you what sucks. The don't act immediately - check how many people complain and then only change things where almost everybody has problems. You will have people that don't like parts of your web application and that is ok. You will never be in a position where the whole world likes your app.
Payment barrier
If your business model is based around people paying for stuff it must be easy. Thats one of the key factors why Apple's App Store is that a success. It is supersimple for Apps to get money from customers. Even small amounts. Almost all Apple App Store users already provided their credit card information when registering their AppleID, so clicking "pay" is painless and only one click. No searching for credit card information, no secret numbers. Just pay. So if you want to sell stuff it must be easy. I personally also like PayPal. They are really working hard making payments supersimple. In Germany you can pay via PayPal without the need to even log into PayPal using only your German Kontonummer / BLZ. Germans don't like credit cards - but they like and know their Kontonummer / BLZ. Seems PayPal did a great job adapting to local customs and even removing a login barrier. Good for customers and businesses. But - the App Store is even simpler because it has one advantage: A closed system.
Login optional - Payment supersimple - even in a non closed system
[1] http://en.wikipedia.org/wiki/OpenID
[2] http://37signals.com/svn/posts/2807-get-started-without-signing-up
[3] http://gettingreal.37signals.com/ch09_Three_State_Solution.php