To be a good web designer your designs not only need to look visually impressive, but also need to be built to W3C web standards, with good titles and meta descriptions.
Just following a few simple rules will make a huge difference, this will also increase your chances of ranking well in search engines like Google.
Years ago web designers would use tables to construct the layout of a website, this method has now become outdated. We can achieve the same results and better by using divs with a cascading style sheet to define the positions.
In terms of seo it's probably not the end of the world, but some theories suggest search engine spiders can access your content easier without them. Using tables to display certain content like a price list or time table is totally acceptable, it's more about the page structure.
Besides the fact people might close your website if it takes forever to load, it's possible that Google and other search engines consider loading times when giving out PR and arranging the order of search results.
Obviously a lot depends on your hosting solution, if your website is sitting on some cheap shared hosting, the chances are you will be experiencing slow loading times, especially if the website is database driven.
It's a good idea to make sure all of your website URLs are short, and contains the main keyword which best describes a particular page. Again some theories suggest this will help with your rankings, it will certainly help to make sure your page is listed in the right place.
Search engines do not like duplicate content, mainly if you use the same article of text on multiple websites, but also if it appears you have a page of content on your website that's accessible with multiple URLS. This is often more of a problem when using CMS websites, the system might list your article on the homepage and a category page, but each time using a slightly different URL.
Each page on your website should have meta titles, descriptions and tags. This will help search engines understand what your page is about, and then they will be able make sure you're listed for correct search terms.
It's also important to make sure every page on your website has unique Meta details, you can use Google Webmaster Tools to find out if you have duplicates.
The final thing to do is check to see if your website validates to W3C web standards, visit this website and put in your web address. Hopefully your website passes, but more often than not you will have a few errors listed that need to be corrected. Thankfully it points you directly to the error in your website by giving you the actual line and character in your website source code.