Vincent Flanders wrote a couple of books, Web Sites That Suck and Son of Web Sites That Suck, which examined the many inventive ways to create an ineffective Web site.
(Note: if the word "suck" offends you, please read Flander's article Is My Web Site Ineffective? instead.)
Flanders' Web site, WebPagesThatSuck.com, provides two checklists to help you answer the question I posed in the title of this post. The first checklist, covering the "149 Mortal Sins That Will Send Your Site to Web Design Hell", is pretty easy to use:
If you check the box for any of the questions, your web site sucks. Period.
I would expect most winery Web sites to fail the first checklist on one or more points. Does that mean your winery Web site sucks? I'm not as judgemental as Flanders makes himself appear. And I appreciate that small wineries have less time to worry about their Web site than, say, Constellation Brands. But I think it's easy to convince yourself that things are better than they seem. No one thinks their baby is ugly.
The second checklist addresses the question "How long can you French kiss before it's a mortal sin?" as it applies to site design. In other words, issues which are in a moral grey area. These are certainly important (like accessibility and standards-compliance), but not immediately fatal.
(You may also find this slightly-modified PDF version of the checklists of interest)
The central theme of Flanders' message is that a visitor's reason for coming to your Web site should be the central influence of its form, a message that The Winery Web Site Report strongly agrees with (see What Is Visitor Effectiveness?). As to the rest, I would hope that these checklists create dialog, either here in the comments, or between you and your Web site designer or webmaster.
Ultimately, it's the feelings people carry about your wine and winery that will determine your ultimate success. I'd be a fool to say that your Web presence is more important than the time spent in making great wine or greeting people who come to visit your winery. But as 2007 comes to a close, I still believe that most wineries have only begun to make effective use of the Web.