With all of these different development elements what should I focus on?
This is a complicated question that, believe it or not, has a relatively simplistic answer. Don't sweat the details; direct your attentions to the "big picture" not each and every intricacy of the process. The "big picture" is the overall look, feel, functionality, and flow of a website. The "big picture" is created around the end users of the website, in other words it is focused on your target market. It is important to remember that an effective website is designed to appeal to a target market and not to appeal to you personally.
Every element of a design is carefully placed and formatted, by the designer, with all the above mentioned elements in mind, specifically your audience. But when you, as the client, suggest a solution ("let's change the text color to brown") it begins to alter the recipe that makes up this "big picture". A change like this may seem insignificant, but after several rounds a multi-dimensional, vibrant design can become flat, dull and lifeless. Once a client steps in and begins to dictate specific details, the design professional is cut out of the process and no longer allowed to develop a creative solution for you.
Am I expected to blindly accept what the designer presents me with?
Of course not, part of the client's role in developing a website is to be the expert on your organization. As the expert you are expected to decide whether or not the proposed design is in line with your audience and marketing goals.
The question isn't weather or not you can provide feedback, the question is how to provide quality feedback. The most effective way of doing this is to present the designer with your concerns and problems. So instead of saying "let's change the text color to brown" you should say "I'm having trouble reading the text," or "My audience is dominantly male, and my products are geared towards outdoor enthusiasts. I'd like a beefier, more masculine color used." Phrasing your design input as an expression of your concerns instead of a suggested change will make a world of difference. When feedback is provided in this fashion the designer maintains creative freedom, while still allowing the client to get the desired end product. An accomplished web designer is an expert in combining intricate details to create the best "big picture" for your organization.
In the end, ensuring that you receive a quality product, that meets your needs, is the responsibility of your web developer. A good web developer will guide you through the vast world of web development explaining your options and helping you make the best possible decisions. With this said, no matter which web developer you choose the communication tips in this article will help improve your end product. If you provide quality feedback, by focusing on the "big picture", your overall web design experience will be greatly improved.