When you are making a design, remember to make sure the colours work together.
The text should always be clear to read, images and filesize should be kept small, but this does depend on the audience. For example, if you are a web design company, go all out on your own design and use THAT to help show that you are competent at designing, I expect design websites to take a little while to load. If your client base is expected to be company or corporate level, then don't worry too much on the filesize of your website's images, because corporate companies generally have some pretty good internet connections, even if it is just for the IT Buffs.
Just make sure it all fits together, and works. The last thing anyone wants to see is the 404, or empty space on your page.