It should introduce your site and explain what it is about. It should not have too many graphics and any that you use should be small enough to load fast. Visitors will not wait long for an opening page to load. Unfortunately, the trend among many Thai designers seems to be to have a big, beautiful picture on the opening page with very little or no text to tell the visitor what it is about. DO choose your colors carefully.
Color is so important, yet some designers and their clients ignore it entirely.
I was recently approached by a potential client who asked me to assess his website for a redesign. Whoever designed it used a dull, boring gray background with black text and a white box to highlight the company name. Your colors should reflect the image you want to project of your business, or they should match your corporate livery. For example, the ThailandExportGuide. And then it uses light pastel colors coded for each section.