I am also big on good and timely communication.
Communication needs time, but can achieve real wonders.
However Live sometimes is stressful and a lot of unexpected events can happen. I agree that it's not a big problem to make quick sure what's going on, but not just for the artist, the person who buys the art also has the responsibility to answer in time.
I already worked with some clients were communication wasn't great at all. This problem definitely occurs on both sides.
After all I suppose artist and client have to match together that something good will be the outcome.
Communication is a skill.
Also how a client communicates the asset he wants to have.
1) I want to have a character in your style
that's quite ambiguous and can mean a lot.
2)I need a character which works like the characters work in "insert game here" with a sprite size of "insert size here" and x and y "frames"
That's already better and tells a lot, howeve rif some of the parameters don't fit together well, it can turn out that the art won't be great, because the restrictions are just plain bad.
For example huge sprites and a really low amount of animation frames will always lead to choppy results.
There are many more ways, but I just wanted to adress that the quantity and quality of communication also can differ a lot.
And one more thing
Indeed it can happen that it's impossible to reach someone via E-Mails, because mails just vanish or an inbox is full, or whatever.
Then it's good to have another way for contact (via a social network, a program, a forum, a different mail account or whatever)
It's also good to agree on a timeframe between messages.
If one exceeds that timeframe, it's a possibility to just resend the last E-Mail, just to make sure, that it didn't vanish.
If I get an E-Mail from anyone, I usually answer it as soon as possible, since that's what I expect from other people too.