It is not possible to ensure every single visitor to your website is happy. There will be a few whose browsers give them errors, or there’s some who have slow connections.
Maybe even ones that do not have Flash installed. Your best bet in any case, is to ensure quick load times, and also ensure that the aesthetics of your pages is right. These two factors will see your traffic smiling as they surf. However, that said, it is not always apparent as to what you need to do to enforce these rules.
The next obstacle is aesthetics. There is only one real way to get this solved, and that is to go to your favorite forum, and ask your peers what they think. Though this may be a scary prospect, you will get a lot of answers back that are very truthful, and enable you to provide a rich experience for your visitors. By taking on board a lot of suggestions, you may be creating work for yourself or your developer, so do not try to do too much. The design of a site evolves over time, due to new features being added, or new advertising methods etc. Your site will eventually grow into the site you want, and the type of site that people enjoy visiting.