The book offers a few reasons, and I think you might be able to come up with more.
1. Winston has been drinking straight gin as his only alcohol for years. His anticipation of wine was that it would be immediately intoxicating (like a hard liquor), and incredibly sweet (like berries... the color it protruded). He never had it before and had only read about it. He really didn't know what to expect, but had built in an expectation.
I think this detail is indicative of what is to come. Winston can taste, so he believes, conspiring against the party. Freedom. However, what is about to happen is nothing of the sort, sorry to spoil the ending.
Wine is an acquired taste. One dose is not pleasing. But after tasting a variety of wines, one becomes an expert and knows how to experience each wine's benefits.
2. Gin, Winston's usual, is taken like a shot. After being thrown to the back of the mouth, it is immediately swallowed, not for taste, but for effect. He didn't know how to enjoy wine. This experience shows that he knows very little about something very normal.