A Channel is based on a 4-panel comic featuring four girls. And basically, that's all you need to know -- it's light-hearted fluff that combines various arche-types that we've seen many times in other series. Run is basically a repackaged Yui, Yuuko even looks like Mio (both from K-ON!), and Nagi is a renamed Koyomi from Azumanga Daioh.
So you know what to expect, and A Channel delivers precisely that. No plot, no conflict, hardly any progression (other than the girls moving from one class to the next).
It's quite derivative, but it's just that I like the stuff it's derived from. That's pretty much all that I can say about it.