1) Seeing the days as sequential leads to numerous textual difficulties. First, you have the age-old problem of the appearance of light in day 1 prior to the creation of the luminaries in day four, which is what led Augustine to posit his famous doctrine of instantaneous creation. In my opinion, attempts to relieve this difficulty by finding significance in the usage of haya rather than bara in 1:14 pay too much attention to the lexical evidence, and too little to the contextual, where meaning is always solidified and made definite. Throughout Genesis 1, “let there be …” in divine speech refers to God’s creative activity, not some sort of refashioning process. If the light came from God himself, as others respond, did God turn this light off “at night?” Did it also contain nutrients causing plants to grow through photosynthesis? Did God also supernaturally suspend gravitational forces so that the universe would not implode without stars, and then suddenly switch these forces back on during day 4? And why are days 1-3 called “days” at all, since it is the revolution of the earth around the (not yet created) sun which marks our 24 hour days in the first place?
In addition, if the days are sequential, how is it that after the seventh day the text can say that “no small plant in the land had yet sprung up” (2:5)? Some people view dischronologization as a terrible problem, but everyone must grant some dischronologization from chapter 1 to chapter 2. And finally, if the sun was not created until day four, was the earth simply suspended without orbit in outer space prior to day four, and then suddenly catapulted into orbit at the creation of the sun? The issue in all these things is not whether God could have supernaturally smoothed over all these points of awkwardness. Of course He could have. The issue is whether the text invites us to read it as though He did. Its not a question of supernaturalism or divine omnipotence but asking, “what did the author intend his original audience to understand from this text?
2) A sequential reading of the days, even when they represent long ages of time, does not square with modern geological and fossil evidence. For example, most scientists believe that reptiles preceded birds, and fish preceded seed-bearing plants. The days in Genesis 1 have birds (day 5) before reptiles (day 6) and plants (day 3) before fish (day 5). These kinds of problems could be multiplied.
3) People who push sequentiality often (though, to be fair, people from all sides do this) seem to me to be coming to the text with certain questions already in mind rather than approaching the text on its own terms. Our starting questions should not be, “what does Genesis 1 say about the age of the universe?” but rather, “what did the author intend to communicate in this text to his original audience (first or second generation Israelites about to enter the promised land)?” “What is the text’s genre, and how does that shape interpretation?” “What is the text’s theological purpose?” “How does it fit into its larger literary context, as part of Genesis, and then the Pentateuch, and then all of Scripture?” And so on.
Though very far from being irrelevant to questions of science, I think Genesis 1:1-2:3 is far more concerned with questions about post-Exodus, pre-Canaan Israel and her covenantal relationship with the Lord. The word “covenant” must be constantly kept only a short distance away to read Genesis 1 well. The main point is: “you know the One who just led you out of Egypt and gave you His law? He is no tribal deity! He is the Creator God of the whole world.” When the text is read on its own terms, within its own matrix of thought and in light of its own purposes, the sequential reading (especiallly the 24-hour day version) becomes much less obviously the “plain reading” of the text, and, in my opinion, a much more difficult, wooden way to read the text.
For these reasons and a few others, I see the “days” as ordered not sequentially, but topically, as part of a literary device (i.e., framework) which is devised to compare God’s creative work to a human work week. This reading does not mean that Genesis 1 is not “true” or “historical” or even “literal” (depending on how define that term), any more than believing in heliocentrism means you deny the truthfulness/historicity of Psalm 104:5: “He set the earth on its foundations, so that it should never be moved.” Rather, this reading seeks to honor the text by reading it as it was meant to be read. Whether it is right or wrong, it should not be dismissed as unhistorical or heretical.
Good resources for further reading on the framework view:
Henri Blocher, In the Beginning: The Opening Chapters of Genesis (IVP, 1984).
Lee Irons with Meredith Kline, “The Framework View,” in The Genesis Debate, ed. by David G. Hagopian (Crux Press, 2001).