The font is horizontally not aligned inside it's box (this is a problem with the font itself). If you add the following mark-up, you can check it yourself:
Label
This shows that the font is positioned too close to the top-right corner. Should be fixed in the font-files themselves.
The problem can be seen in the screenshot:

@franz-josef-kaiser is this still present in 2.0? New font exports were made for this version
@kremalicious Can't confirm so far - have to look again at it. Did you check it yourself?
If you look Font-Awesome web site, you can clearly see that the font is not vertically centered to the text. Just an example in that page, look the text "70 New Icons in v2.0", it's about 7px on top and 3 px on bottom compared to icon, it should be 5px and 5px to look good.
Icons are purposely not aligned to the middle. If you have a problem with the specific alignment of any particular icons, just open a new ticket. This should be quite a bit better in 3.0.
Icons are purposely not aligned to the middle
... and would you mind explaining that _purpose_ ?
Because alignment is relative to the text. Aligning to middle will render too low. Aligning to baseline will render too high.
That would be the _purpose_.
@davegandy the issue here was actually horizontal positioning, not vertical ...