#1 The browser size changes from user to user. Make sure that it is consistant on different size monitors.
#2 No matter how much they are annoying there should always be a form. Try to encourage people to use the form even if it is a 3 line form. Get that email address.
#3 The constant landing page subjecting viewers to accept too much information. The home page is supposed to introduce you to the company not push you away.
#4 Cross browser capability, this is a constant disturbance to doesn’t allow every user to enjoy your webpage in the manner that it was meant.
There are so many just wanted to touch on some of the one most people are aware of.