Posts look different: the most immediate and obvious change is that line-spacing has changed. Quotes are now indented a bit more, too.
Old (from here):
What just happened?
SE have implemented a new formatting regime which has also included changes to spoilers (which now require a click rather than hover, so they're more consistent across platforms).
All the gory details are on MSE: New post formatting
This question and answer is provided purely as a pointer to the main article as not everyone will see the Featured post link.