Ok, I can't resist throwing in a couple cents:

Art, (all following text is opinion) is anything which appeals to the emotional, uses aesthetics rather than pure functionality, to elicit a response of any kind. I consider design art. I consider the housing of a computer hard drive art if it is created to appeal aesthetically rather than house a main frame in the most functional base way.

Written language is a form of visual communication, the complexity and sophistication of which strives to mimic the both literal and implied information we are able to deliver in vocal communication which uses a combination of inflection, facial expression and other body language to communicate ideas and thoughts which are not necessarily contained completely within the words used. The rules of written language are complex for that very reason, the more complexity the more minute detail it is able to convey.

When the two collide sometimes the rule of one compromises with the rule of the other. i.e. Aesthetics compromised to communicate the text clearly and concisely, or the rules of grammar compromised in order to elicit a response that may communicate the idea intended better in the "feeling" evoked as opposed to pure written communication. i.e. the difference between writing, "BE SHOCKED!" and finding someway to make the viewer FEEL shocked.

As far as "lines", I believe they are drawn by the individual. In the world of marketing, I suppose the success of the product being marketed would be a barometer of the social acceptance of the "bending" of either rules.

I think that all lower-case is a fad. But I could be wrong. I don't personally find it appealing, but you can't argue with its general acceptance. In language and societal evolution majority always rules.

Anyway, very interesting thread. I hope nobody's feathers are truly ruffled, it's a worthy discussion, but nothing that needs be taken personally.