The font is already different, there is a larger left margin on block quotes, and generally code blocks have been syntax highlighted in an obvious way. However, these differences are most obvious when they are placed side by side and depending on the content/size of either they can be difficult to distinguish with their current styles when placed separate from one another.
I agree with @jjnguy that too many styles can make a page look noisy, but they can also provide clarification when the differences are obvious, but minimal, and in keeping with the overall tone of the site.
@Cody Gray's sample from Super User with the thick left border is a great example of what I'd like to see. It could look something like this:
I like this for many reasons including:
- Standard style used on many sites which helps make the meaning obvious
- Minimal impact on page "noise"
- Easy to implement (can be done in one additional CSS attribute)
I'd hate to see large quotation marks, italics, a large leading letter, bubbles, background images, inverted colors, or any of the other potentially horrible designs I've seen. Although, we could just go all out and do something like this: