In the past, word walls were meant to show only words. But, looky-looky at the uses of the wall. I have a word wall that is EVER changing. Mostly, it has the high frequency words, vocabulary, and specific words for writing. The kids use it, however, the wall is not very differenciated. The words are based mostly on the LOWEST kids, and doesn't help with most spelling because there isn't enough room!
My AMAZING 2nd grade team (9 teachers in all) came up with a "Walking Word Wall." They are laminated file folders with alphabet on them. Basically, they are MINI word walls that we keep in our reading notebooks:)
When a kid needs to know how to spell words (which they are ALL different words), they bring me their Walking Wall and I write on the file folder in Sharpie! This cuts down the repeated questions on their words:) Enjoy the tip!