How to let a journal know that its submission process is very painful? I have recently decided to no longer publish, or at least publish less, with a well respected journal in my field. How do I let the journal know in a meaningful way that the submission process is very painful?

The required Microsoft Word and LaTeX templates are extremely dated and do not match my (or anyones) workflow. The review time (6 months), number of reviewer (3-4), number of rounds of review (3-4), and delay from in press to available online (8 months) are all too much. The lack of communication is extreme. The online system tells you if the manuscript is on your desk or the journals desk and nothing more. Editors do not respond to queries other than to tell you it is being reviewed.
You may get the best response by using informal channels.  If you know the managing editor of the journal (my field is small enough that I often do), you might send an informal email, or even just express your concern in person.  As Ben Norris mentioned, the key is that you communicate in a calm, rational, and deliberative way.