Blockquotes are often shown in italics. If properly done (I guess), any italics in the actual quote is then "reversed", becoming upright. I generally have a much harder time discerning upright type in a block of italics than type in italics in a block of upright, often having to look twice to make sure the text I just read was actually upright (I assume I'm not alone). In short, upright inside italics always seems to interrupt my reading.
Why is that?
Example (may or may not be a good one depending on the font):
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Proin enim elit, porta et pulvinar a, molestie vel dui. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Fusce congue, augue non dapibus dignissim, justo diam luctus justo, vel eleifend urna dolor nec quam. Sed quis varius velit, ut lacinia risus. Curabitur efficitur ultrices tellus nec finibus.
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Proin enim elit, porta et pulvinar a, molestie vel dui. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Fusce congue, augue non dapibus dignissim, justo diam luctus justo, vel eleifend urna dolor nec quam. Sed quis varius velit, ut lacinia risus. Curabitur efficitur ultrices tellus nec finibus.