I happen to be a bit persnickety about some things, and it's always bothered me, just a tad, that most challah covers have the words "Shabbat v'Yom Tov" embroidered on them. I always thought it would be nice to have one that mentions Shabbat, and a separate one you keep in a drawer and only take out a few times a year on Yom Tov. (Obviously you're not going to need one for Passover, so that leaves Rosh Hashana, Sukkos and Shavuos.)
There actually are a few out there that say only "Shabbos," and I just came across a series of challah covers designed for individual holidays throughout the year: Rosh Hashana, Sukkos, Shavuos and even Purim. I have a feeling not everyone is prepared to spend $50+ for a challah cover that's only going to be used one or two days a year, but they definitely make great gift ideas for the type of person who has everything. Or, say, as a wedding gift when you know the couple is going to receive challah covers from other guests.
Kaftor Vaferach, a fabulous embroidery company in Jerusalem, has special challah cover designs for Rosh Hashana, Purim and Shavuos.
Ronit Gur, a top-notch Judaica maker in Northern Israel, has a few "multi-purpose" Yom Tov challah covers. The letter embroidery says simply V'samachta b'chagecha, which applies to all of the holidays, so you can get a bit more use out of it. The Ronit Gur selection for Shabbat employs various texts: L'ch'vod Shabbat Kodesh (see photo), Shabbat Shalom, Hamotzi Lechem Min Ha'aretz, U'v'yom HaShvi'i Shabbat Va'inafash and a very quaint one that reads Lecha Dodi Likrat Kallah Pnei Shabbat Nekabela.
Even for those who are not about to shell out money for holiday challah covers, some might like the idea of a challah cover that says just Shabbat if they feel that Shabbat v'Yom Tov "waters down" the kavod Shabbos.