1. High Quality design: Now what constitutes a high quality design! It is again subjective in nature. A design which looks appealing to one, may not look the same to other. But still there are few necessary things which a website design must have:a) Balance and Unity – There should be balance in different design elements of the design. All the complex as well as simple design elements should be mingled well with each other and integrated as a whole.
   b) Focus – Main emphasis should be given on headings, where you would catch the attention of the readers. The main points should be properly highlighted. Effective Call to Action buttons must be placed in the area of good visibility so that more sign ups/conversions can take place.
   c) Contrast – There should be proper contrast between the background color and the typography. Not just this, but all the pictures, patterns, textures should be mingled well with each other in terms of color contrasts.
2. Special Attention to Details: Even minute details should be given importance. Because small small things will make you to stand out from the competition. You need to think and plan out, whether particular heading should be underlined or not, type of font to be used, etc. Always think what can be done to improve the design. If you need some help, go for usability testing or call for forum participation to see what others are saying about your design. People get bored with the same old design – give them something different, unique, and attractive, and they will come back and visit.
3. Multi-Stage Approach to Web Design Process: Website Designing should be perceived as multi-staged process. First step should be complete conceptualization, undergoing the thought process, performing brain storming activity and proper planning. Then go for creating wireframes, after that comes the implementation part i.e. creating design element, content, PSD to HTML Conversion etc.
4. Content should be well communicated and grammatically correct: There can be nothing more worst about the website than poorly written, grammatically incorrect, misspelled content. This annoys visitors to a great extent and can earn a bad reputation for your website. You may hire content writers if you yourself can’t write well. Run through your content a few times to confirm that everything makes sense, is interesting, and is spelled correctly. If not satisfied, make use of check spell and grammar tools.
5. Excellent Usability: If website usability is poor, then people can easily become frustrated and leave your website. You need to ensure few things- simple and easy navigation for your visitors, less information requirements from your side, don’t ask for registration every time and other similar things which affect the usability of the website.
6. XHTML and CSS Validation: You need to do due diligence before the launch of the website. You need to validate your code. These days people access Internet through various devices like PDA’s, mini laptops, smart phones and lots more. You need to ensure that your website looks almost the same and is accessible through all these devices. Also you need to ensure that your website has cross browser compatibility. The mark up should be W3C standards compliant.
7. Optimization of Web Pages: Website optimization is another crucial factor that must be taken into consideration while designing a website. Also professionally designed websites should be designed with a solid SEO friendly foundation. Slicing play an important in web page optimization. Large images should be sliced into small for easy loading. Similarly to images, audio/videos should be optimized. CSS navigation guarantees that your website navigational text is crawlable by search engine spiders. Don’t make use of heavy design elements like flash etc. because they create problems while loading. Optimize your web pages by including relevant keywords to the content and title of the page as well.

These 7 tips altogether make the best of attributes of a professional website design. All of this cannot be learnt or taught in single day. Practice makes man perfect. So do lot of practicing, and develop professional web design in real sense. There may be some others attributes too. Probably, you could come out with few, after reading this.