Ian Murray puts forward two views of what constitutes expository preaching:

1. Where the preacher “…confine[s] himself to the text of Scripture, and … make[s] the sense plain to others…”
2. The same as the first except this also insists upon working through a book or passage of the Bible consecutively.

Murray is addressing the view that the second way of expository preaching is right and the first, applied to texts of scripture which are unrelated from week to week, is wrong.

There are several obvious benefits to preaching through entire passages/books, and Murray details them briefly. However, his intent is to focus on the disadvantages of such a style of preaching.

First, not every preacher is gifted in this style of preaching.

Second, preaching isn’t necessarily about covering as much of the Bible as possible – it’s about meeting the current needs of the congregation.

Third, working through an entire section of scripture consecutively can easily become a lecture rather than a sermon.

Fourth, the consecutive style tends away from being memorable due to covering too many ideas (rather than picking one or two main ideas from the passage). Murray explains that this is why Reformed preaching is often considered dull.

Fifth, consecutive preaching  does not fit well to an evangelistic style. Murray says that “preaching to heart and conscience commonly disappears.”

Murray finishes by saying that these points should not stop this style of preaching from happening, but that preachers must use it at the right time along with the one-off type messages in their right time. He is eager to emphasise that both consecutive and individual/isolated verses can be expositonally preached, and that all preaching must be exposition of the Bible.

http://thegospelcoalition.org/Expository_Preaching_Iain_Murray.pdf