First, just what is a filter thread, and is it an essential when camera hunting?

Second, when I was here a couple of months ago, asking for a recommendation for a good family camera, the Nokia D50 was recommended. Now, I'm comparing it to two other camera's that other people have recommended, the Canon Digital IXUS70 and the Canon EOS30D. There are some serious differences in prices here, and I'm flummoxed. Any opinions anyone?