It is not uncommon for new web designers to spend hours upon hours focusing on the way their website looks. After all, visual appearance is an important element of website success - if your site is not visually pleasing, people will not stay and look at it. However, it will not matter how great your website looks if your users have a hard time using it - either in terms of finding the products they want, navigating from page to page, or even getting to the site itself.
Check to make sure that your site works when you type in the "www" before the URL, and when you don't. Different users have different habits when it comes to navigating the World Wide Web, and you want your users to be able to find you either way. If you have a reputable hosting site working for you, then this should not be a problem, but you will want to test it either way. After all, you are the one in charge of making sure that your site is up and available.
If your host does not have a high uptime, then you could be spending a considerable amount of time waiting for help when you need it. The pace of change on the Internet is unbelievably fast, and you don't want users leaving your website to go to a competitor that has a better host. If you are depending on your website for income, check reviews of your host before signing up.
It is OK to have third-party applications running alongside your website; in fact, this can help make your site a more fun place to navigate. However, if you do not use a sandbox, then your website is vulnerable to any attacks that may come through (or even from) that other application. Use a sandbox to insulate your site from danger.
Make sure that you can see your website in all of the major browsers. If you can't, then you are missing out on client traffic that uses that particular browser as a default setting. There's no reason not to appear on them all.
If you have the knowhow, then you need to run periodic checks for dead links or code that is not working. If you are running alongside third-party sites, this sort of code can sneak in - dedicated hackers may even be able to do you the "favor" of adding it. If you do not have the knowhow, make sure that this maintenance is part of your hosting contract, or pay a friend to run periodic checks. This will keep your site user friendly.
Also check how your site looks on the big mobile browsers. Nowadays people are doing more and more of their surfing on mobile phones. If your site is not available, or even user-friendly, on these platforms, then your potential customers will find a competitor site that works better and you will lose revenue.
Keep those certificates up to date. That is a simple matter of staying on top of your calendar.
Run through your htaccess file now and then, checking for any errors that have to do with permissions. If you need to delegate this to the person who is checking your code as well, that's fine, just get it done.
Users want content. If you don't provide content that is convenient, no one will go to your website.