You know your wife better than anyone. Don't panic when it comes to writing something sweet in a Valentine's Day card. Avoid cliches and be specific. If you could say the same thing about any other woman, then you need to think harder. "You have beautiful eyes" isn't going to cut it but "I love the freckle on your left knee" is sweet and thoughtful.
I Love Us
Many Valentine's Day cards say, "I love you" but go a step beyond that and say, "I love us." Tell her why you love your marriage and partnership. Express that you love not only her, but your relationship. Show appreciation for the way the two of you work together and the life you've created. Write things like, "I love being married to you" or "I love that we found each other" or "I love being one half of this marriage." It not only shows that you love her, but that you respect your life together.
Thank her for everything she does. It could be big or small. Thank her for washing your socks, teaching you the proper use of a semi-colon, or raising your kids. Thank her for being her. Let her know you appreciate the effort she makes in your relationship. Write, "Thank you for teaching me how to be a better man (and how to fold laundry)." If you have kids write, "Thanks for always being the bad cop" or "Thanks for letting me be the good cop."
Is there a movie the two of you love? What was your wedding song? What's her favorite book, movie or song? If you don't have a way with words, quote meaningful lyrics, sayings or movie scripts. Find quotes from romantic comedies she watches or erotic novels she reads. If you quote her favorite author, for instance, she'll be touched that you remembered and appreciate the time you spent looking up quotes and choosing the right one.
You're a Great Woman
She's more than a wife and more than a mother; show her that you appreciate her as a woman. Write, "You're such a good-hearted human being" or "You're a sexy, intelligent woman" or "To the funniest woman I know." If your wife has a passion or hobby, praise her for it. Tell her she's the best artist, chef, yogi, or magician you've ever seen. Write that her paintings put Monet to shame. Write that she should have her own cooking show.