One of the most common problems and simplest to fix. You have to remember that while you want your website images to look crisp and high quality, the larger your website is, the slower your site will load (particularly on mobile phones).
Having a video run in the background of the front page of your website might look great, but you're going to need to compress it so it's at least under 1MB in size. You also should defer loading large content like images and video, so you don't frustrate your users with long wait times.
2. Website isn't Compressed, Minified or Cached
Running on from the first point, every website, even well written ones, should take advantage of compression (like gzip), minification and caching systems and content delivery networks (CDN) to make the user experience as fast as possible.
If you've a slow website you may well be losing customers & business. This article explains more about that impact.
You're also going to particularly see an impact on mobile users (63.4% of your visitors) - phones aren't as fast at rendering a website as a computer, so your website should be written to accommodate.
3. Not Accessible
According to the latest available UK government report a surprisingly large proportion of users use assistive technology (around 29%). These may be users with visual impairments or who have difficulty interpreting screen content. There are so many websites out there that just don't consider those needs, which must be incredibly frustrating. Images should have descriptions (alt tag), links should have alt tags to describe where they will take the user. Every decent content management system will provide the functionality for this, so make sure you're using it.
4. Text is Too Small
The standard text size on most web browsers is 16px tall. This is surprisingly big - don't make your users squint, or have to zoom (I'm looking at you FaceBook with your default text size of 11px), or abandon your site just because it's not readable. And that leads to point 3...
5. Contrast Ratios
As well as a design consideration, this relates to accessibility. You should have enough contrast between colours to allow all text to be easily readable. Remember 8% of men and 0.5% of women in N. Europe are colour blind. Reds and greens are a particularly bad combo (red and green should never be seen without a colour in-between). If you need to to check that you have enough contrast, this handy tool can help you.
6. Not Secure (https)
There's no excuse for this one these days. Every website should be encrypted (httpS rather than http). Let's Encrypt offer a free website encryption service which most modern hosting services can easily install in a few clicks.
Why does it matter? When a user submits any information to, or browses a site, information is passed from the users browser to the web server via their internet connection. Now, let's say for the sake of argument you're in an airport, or cafe on public WiFi. If someone were able to intercept your connection they could see all the information you are getting or sending to insecure websites. Any personally identifiable information could then be used to steal your identity, money or login information.
How do I know if a site is secure? Most browsers display a little lock symbol beside the website address. If it doesn't have one and you are being asked to fill in a form or submit information, you should avoid doing so.
7. Too Vague
If someone who had never heard of you, and had no idea what you do, were shown your website, would they be able to work out what you do? How long would it take them to do so? If the answer is less than 5 seconds then you need to review your messaging. I recommend all my clients do the free 5 minute marketing makeover by Story Brand. Which will really help you understand why messaging clearly is so important.
8. Not Built for Mobile (responsive)
63.4% of websites are viewed on mobile devices (phones and tablets) in 2019, so if your site is not designed responsively around that majority of users, you're missing out. Also, your search engine ranking will be affected as a result.
Google announced last year that they were penalising sites that have popups (modals, interstitials, overlays). Rightly so. You still can't go on the internet these days without coming across this kind of page. You're 10 seconds in, or half a page down, and all of a sudden you can't see content because there's a popup asking you to sign up to the latest newsletter. There are lots of better more honest ways to engage with your users than forcing them in to signing up for your e-mail list.
If you've a genuine service that's relevant to the user, and you're offering something of value in return for their email address, then they'll sign up. Just don't bully your users, doing so just creates a mild frustration and devalues your brand.
10. Badly Coded
A lot of people use WordPress because it's wonderful, free and open and extensible and accessible, but use (or are sold) an 'off the shelf' multi purpose theme, probably because the demo content looked great.
I've done this for clients in the past who don't have the budget for a bespoke site and I can tell you that for all the short term gain, it's long term hassle. I'm not saying this is always wrong, some people run entire web design businesses this way and that's OK.
Common problems are that the codebase becomes obsolete over time, the core system will update but the theme gets left behind, becomes insecure, hackable, or worse creates errors on the front end of your site or breaks it entirely.
The other issue is that pre-made themes are often bloated. Remember the people who make themes are trying to sell them to as many people as possible, so they need to build in more options than any one site will ever use. So those themes will be bloated with superfluous code you may never use.