Everyone brings up a good point about Willow being very technical and less religiously inclined. However, she does still claim to be a practicing Jew, and I always thought the show used Wicca more as being synonymous with witchcraft, rather than paying much attention to the actual pagan worship aspects of it. I do remember Willow being dismissive of the Wiccan group at the University, but I always thought that was because they preferred to focus on bake sales and female empowerment, rather than actual spells.
When I say she adopted the Wiccan way of life, I guess I'm taking a clue from her change in dress and how she adorns her living space. She does go from fluffy sweaters to the weedier look adopted by Tara, complete with neopagan jewelry. Plus she becomes somewhat protective of the term "witch" around other people, much like how she reminded people she's Jewish in the earlier seasons. Also, she gets annoyed with and tries to change people's perspectives of witches on Halloween, and later on casts Xander the side eye in "Once More With Feeling". I take all this to mean she's not just using the magic and passing on. At some point she is feeling a sense of belonging to a group and is taking up for it.
Willow may not be dancing at Stonehenge, or constantly praying to the Goddess, but does no one see her giving a cheeky "Happy Solstice" on the day, or taking the time to utilize a particular spell? Maybe even distribute a nice concoction brewed just for that day? Willow always seems to champion causes and ideas that go against the grain of mainstream society, and she revels in sharing her alternate perspective with her friends. Does no one think she'd do the same on the Solstice?