We generally concentrate on 
1. Performance Testing
2. Stress Testing
3. Load Testing
4. Volume Testing

we concentrate more on login page & some standard pages
while doing Web Application Testing

rajush u r correct but u missed
security testing and link testing (deadlinks & broken links)

Gen rally for web site testing the most important factor is
Look N Feel Issue for that site how your web site look.

1. THE ALIGNMENT SHOULD BE PROPER.
2. PAGE BACKGROUND AND FOREGROUND COLOR SHOULD BE PROPER.
3. PAGE LAYOUT SHOULD BE PROPER.
4. FONT SIZE SHOULD BE PROPER.
5. TAB NAVIGATION SHOULD BE PROPER.
6. THE PAGE IS FREE FROM SPELLING MISTAKES.

Thanks for your answer Devendar but I would like to correct
some thing in your answer. What you have mentioned is right
but it all comes under low priority.We should concentrate on
high priority while testing the web application we need to
concentrate on lot more like the links are properly
working,page navagations, Browsers, whether its taking the
right password and the password is being encrypted or not.
And also need to check whether its taking the right input
and proving the correct output and lot more things under
security stuff.

1:performance testing
2:link testing
3:usability testing