I quite often see fundamental design problems being solved in the wrong way when it comes to website design. Here are some of my pet hates.
Telling me to scroll down
If you need some sort of bouncing arrow or other funky gimmick to get me to scroll down your page then you might be better reducing the size of your huge hero image or adding some enticing content which makes me want to scroll down. Or better still, put the relevant information at the top of the page. Lose the silly bouncing arrow.
If you have to put some sort of animation in place which tells me that your page is loading, then your page load times are taking too long. So instead of adding even more weight to your page simply reduce the amount of time that it takes to load it in the first place, then you won't need to convince me to hang around whilst it loads. Consider removing fancy design effects such as box shadows, parallax scrolling (no you really don't need it!) and optimise your images. Test your page load times on multiple devices and network connections to see what you can do to improve it. Remember that anytime you have to convince people to hang around whilst your content loads is a risk that they will just move on.
Huge hero images
We have all been guilty of this one because, as designers, we like to be image-heavy and what's nicer that starting your page off with a lovely big image? Well it might look nice but it can still be bad design. Remember that good design isn't just about looking good. That's too shallow. Good design takes into account performance, getting the right message across and all that kinda stuff as well. So think about your audience and stop tickling your design ego.
I suppose this takes into account all of the above. If you don't want clients to use off-the-shelf themes and instead hire you for a custom design then don't give them a design that looks like an off-the-shelf theme! Does that really need saying!? I guess it does. Trends die. They die fast! As soon as you launch your trendy design it will look dated. Do some research into your client, their history, their intended market and design to that. This is why they are hiring a professional designer. A good design is timeless. A trend isn't.
There are plenty of other things that I could add to this list but I'll perhaps write a sequel. I don't want this to sound elitist because I'm just as guilty as you for making the above mistakes but the key thing is that we all learn from them so that we can design better websites for our clients. We are professionals and it's our duty to do the best work that we can so please don't take the above to heart too much, just think about the designs that you are producing and take the above into account. Ask the question, "Is this really the best for my client and their target market or am I following some sort of trend?".