Jonathan, Kasey & Mike have been talking about meter & rhythm. Recommend an interesting study:

The Strict Metrical Tradition, by David Keppel-Jones. The author attributes the amazing success of iambic pentameter to just a few technical innovations: basically, by maintaining a strict syllable count (with the exception of feminine endings & elided syllables), and combining that with 3 "radical variations" (as opposed to the simple substitution of a weak for a strong syllable (pyrrhic) or the substitution of a strong for a weak (spondee)) - these being, first, the inversion of an iambic foot (/~ instead of ~/) - but used in specific, controlled places in the line - and the 2nd and 3rd what he terms the "minor ionic" & the "second epitrite" respectively. These two are measured in sections of 4 syllables: ~~// (minor ionic) and /~// (2nd epitrite) - and are also located in specific places on the line (usually at the beginnings of lines). Most of the book is taken up in explaining the rhythmic implications of these innovations & a detailed historical survey running from Spenser & Sidney to the Victorians.

One of the general lessons to be drawn from this kind of study is the symbiosis between "strict order" & "radical variation" - the variations are effective because we are, necessarily due to the meter, attuned to very slight stress emphases.