About popups/tooltips/hovercards: I considered this approach, but decided to go with this sort of "inline" expansion for the following reasons:

1. A popup obscures the original text. This interferes with context, albeit admittedly to a much lesser degree than full-blown navigation, but still.

2. Multi-level popup hierarchies just don't look good, in my opinion.

3. You must dismiss a popup to continue, whereas with inline expansion you can just as well keep the expansion and continue reading.

4. The hover approach doesn't work as well for touch-devices (i.e. without a mouse).