Formal poetry typically includes the following features
The rise of free verse made the line-break a more flexible device. Numerous experimenters explored the potential of layout. Some poets used ragged-left as well as ragged-right formats - not as the Alexandrians, Herbert or Lewis Carroll did (making a shape on the page), nor to emphasise the rhyming pattern, but to guide meaning and control rhythm. At the same time ee cummings launched his attack on capitals. Yet the ghost of formal verse still haunts even some less conventional modern poets. Here's stanza 7 of "Echo-Poem" by Medbh McGuckian (from "Marconi's Cottage", 1991).
I touch the foxglove
Curtains with my tongue,
And find them salt.
They fall into five
A chessman into
The hollow course
Of the window.
None of the poems in McGuckian's collection uses prose line-breaks. Though no standard forms are employed, nearly all of her poems are "block poems" - sometimes short lines, sometimes long; sometimes 2 lines per stanza, often (as here) 8 or so - but within each poem there's little or no variation. Each line begins with an upper case letter. Punctuation and grammar are conventional. There are no puns or end-rhymes - little that depends on the arbitrariness of aural signifiers. And yet, arbitrary components of visual features (how long a set of words is) determine the layout. The thought is unrestrained, but the layout isn't.
Though with free verse there's no obligation to use line-breaks in specific places, there's pressure to use one before the prose line-break arrives. Free verse's default line-break coincides with the end of a sentence or clause. Deviations from this norm attract the reader's attention in the way that deviation from meter did in formal verse. How successful are the line-breaks in the cited extract? The break after foxglove makes Curtains more effective, but what is the purpose of the unexpected line-breaks in They fall into five Acts, flinging A chessman? Indeed, is there any intended purpose at all? Presumably so, because there was no obligation to use line-breaks. Some poets (Bernstein for example) say that they use weak line-breaks like speed bumps to disrupt the reader. But when there are so many line-breaks that try to attract attention it's tempting to ignore them. And what's the purpose of the initial capitals and the regularity of the stanza length? The visual, almost incidental, components of old verse have been left untouched, though one might have expected "the most white-hot Irish poet since Yeats" (Calvin Bedient, Parnassus) to have broken free of formal straight-jacketing. After all, D.H. Lawrence managed it.
McGuckian is not a special case. Many free verse poets use visual formal devices though their necessity has gone, and in some cases so has their purpose. Perhaps they are used merely to confirm that the piece is indeed poetry rather than prose. After all, free verse has jettisoned so many characterising features of poetry that maybe it has to cling to the few it still has.
Though free verse has inherited many conventions from formal verse, they've not all been with equal respect. While some have been developed, others have died away. Sometimes new means have been substituted to attain similar ends. For example, formal line-breaks and meter often provided expectation and an anchor point in each line. Halliday and Hasan amongst others showed how cohesion and coupling are achieved by different methods in modernist poetry. But by not adopting the prose line-break as its default, free verse has copied the appearance of formal verse without a rationale, confusing the coupling and anchoring mechanisms.
This damaging conservatism amongst modern free-form poets serves as a warning as we enter the hypertext era. Scroll bars free us from the restrictions of the page edge, text can move and transform as we read, allusions can be solidified into links, static forms can be replaced by dynamic connections understood best by reference to the mathematics of topology and graph theory rather than arithmetic. But this potential won't be realised if poets retain old forms merely for form's sake.
Amongst the many opportunities that the World Wide Web offers writers, one rarely mentioned is that coloured text and background is as cheap and easy to produce as black-on-white. It's instructive to compare the potential of colour with that of other devices. By careful selection of a word's colour extra graduated meanings can be generated - lines can fade to a whisper. Colours, like sounds, have an emotional effect of their own, and distant words can be coupled by making them the same colour, just as rhyming words can be coupled for effect. Adding colour (like adding line-breaks) doesn't mean that anything else has to be removed, so there's nothing to lose by adding it.
However, colour is unlikely to become popular with mainstream poets because (except for those who are synaesthetic) it's not a core property of language or even of writing. Sound is, though some poets claim they pay no attention to the sound of their work. Punctuation is less central (though pauses are natural), fonts and line-breaks less central still. Whereas punctuation derived partly from transcribed speech, font effects and line-breaks are a consequence of the rise of the written word, and were used in graffiti, gravestones and advertising before free verse appeared. There's little mainstream tradition of using colour or font variation in poetry - it's seen as attention-seeking gimmickry. The free verse line-break is different from these effects only in that it was used (of necessity) in formal verse. In other respects line-breaks are as peripheral as fonts and colours; capable of expressing nuance, but essentially a visual device. The resulting "block poems" and leading capitals may please the eye (as might a nice font) but the thinking mind is (or at least should be) harder to satisfy.