When Harry Met Sally is perfect, and I will fight anyone who tries to argue otherwise. I love this movie. You could approach me at pretty much any time and I will be willing to watch When Harry Met Sally with you. That is a true story.
When Harry Met Sally is concerned with a specific question - Can men and women be friends? The answer it arrives at: No. They cannot. That, admittedly, is a little bit odd to me - It says very strange things about the nature of relationships - but I'm willing to overlook that oddity in the face of such overwhelming quality.
When Harry Met Sally has dialogue that sounds like things that real people might say. Harry and Sally's romance is based on actual interaction and qualities of each other that they enjoy, not some intangible feeling of magic. It's funny. It develops it's characters through their conversations and interactions with each other, but not through heavy-handed metaphors like the romance movie motif of Sleepless in Seattle. Billy Crystal is wonderful and charismatic, and Meg Ryan is relatable and real and oh it's just all so wonderful. There's just nothing wrong with this movie. I enjoy watching it, I don't feel like I have to sit through it. The characters feel like real people with believable flaws - Harry's cynical and thinks he's smarter than he is, Sally's structure allows her to push away her feelings. It's a story, and believable, and it's everything a romantic comedy should be.