I’m not a web designer myself. 
That’s why I’ll tell you which design mistakes
are repeated over and over again.

Making a cute web site
 

Don’t make the web site just cute. You main goal should be to influence users with the power of arts. Listen to the business people carefully — they want their users to be persuaded doing exactly what? In a web shop, think about what can motivate users to buy things in your shop, not in the neighbour one. In an ad-based revenue model, ensure the users would come back over and over again. Send an emotional message, if you need. Build trust, if you must. Charm them with elegance. Overwhelm them with power and energy. Disarm them. Inspire them. Make them laugh. Make them cry. Embarrass them. Soothe them. Praise them. Make them curious. Make them feel your respect. Manipulate users to help achieving your business goals.
 
 

Not owning design
 

Late change requests tend to be placed in any free area, without re-thinking and re-evaluating the overall impression of the site. Protect your site from scars of change requests. Insist on a new composition to incorporate the new feature in the optimal way. Educate decision makers about negative consequences not doing so. Fight for your baby, own your design.
 
 

Ignoring real data
 

Dummy screens always look so harmonious. Colors fit together, texts fit to their areas, no part is pulling the picture to a side, the focus is flowing perfectly. The only problem is that your users won’t ever see the dummy screen, but the real app filled with the ugly, chaotic, dirty real content. Ensure to get the real random content examples and create your design with them. Test your design with extremes (absence of content, too much content, bad content quality).
 

Not reusing controls
 

The app looks fine in the minimized mode. But let’s look at it in fullscreen. Well… not bad at all, but let’s just spread the controls a couple of pixels apart, for a little better look… Stop. You’re doubling development efforts here, achieving another 2% improvement of design quality. Better pay attention to other areas.
 
 

Reusing controls
 

Coverflow looks cool. We’re using it to select from products. Fine. But we also have another item lists: site navigation; selecting day, month and year of the birth date on the registration form; choosing credit card in checkout… Reuse the coverflow control? Obviously not. While it would undoubtedly save development efforts (unless you change a couple of pixels from usage to usage, see the point above), it would often ruin the overall web site impact.