The biggest mistake most wineries make online is to establish a website, put some stuff up, and then let it molder for months if not years. I routinely get tasting samples in the mail ...So I’ll go to the website for info, only to find that the vintage hasn’t been updated for two years!... when you deliberately go to a website expecting something new and useful, only to find a bunch of old, boring stuff, it’s an insult. You actually feel resentment to the company for being lazy and uncaring and unprofessional. Not good.
The top error for winery websites remains a lack of freshness, from home pages which haven't changed in years, to a lack of information about new vintages, to "News" pages where the last update was more than a year ago.
A related mistake is to throw out the "old, boring stuff" on your website wholesale, instead of archiving items which have long-term value to visitors. Disk space is cheap and search engines are your friends. Someone just might want to read about that 10-year-old vintage they pulled from the cellar for tonight's party. Ideally, your website has information about every vintage you've every produced.