As one who almost ALWAYS looks to the Web for a product FIRST, both in my professional capacity and in my hobbies, I will list the things I will NOT tolerate when I am surfing with money to spend:
1) Slow! I have mega-high speed at work and 36K dialup at home but I will not tolerate waiting. Keep the primary pages simple, clean, and FAST. Leave the heavy graphics load for very specific pages, ideally with a warning as to the size.
2) Poor index, site map, or search functions. I know what I want and the first question I have is whether or not you have iti (or similar). If that question isn't answered in 30 Seconds, I AM GONE!
3) Too many "phone for price" or "price on request". I am there to shop and if you wont post your prices, I don't have time for you.
4) Information (links to other pages) too hard to find - they should JUMP right out at me when the page opens. Remember, I know what I am looking for - I probably got there thru a search engine and there's 7,000 other pages I can look at if your's doesn't grab me.
Now I will say there are different styles of Web pages: some for entertainment, some informational, some aimed at sales. Make sure your Web page caters to those you are willing to spend you bandwidth on!