Since developing a good website is quite a time consuming process, you would ideally want to work with someone that is able to meet these targets on time. People sometimes tend to make a ton of promises but are unable to keep them up. Hence, this might end up causing them to reschedule and make changes. Ideally, you wouldn't want your website designers to be in this category. Setting realistic targets and having the kind of designs that are practical is highly recommended to ensure that you too are able to make plans for the site in accordance to its finish date.
These are just few of the qualities of good website designers. There are plenty more to keep track of, but you wouldn't necessarily worry about them right now. The important thing to keep in mind is that you are able to get someone that can get the job done for you without much of a problem. It might seem a little tedious at first to get such a designer, but after trying for a while, you can definitely get someone that can take care of these things for you.