The German site, bluray-disc.de, has provided details of the bonus content of the German Blu-ray release. These appear identical to that of other releases, where they are included on the same disc that contains the 2D version of the film.
1). Commentary by the film makers.
2). 4 Deleted scenes with commentary by director, Marc Webb.
3). 9 Additional deleted scenes with commentary by director, Marc Webb, including "Peter meets his father".
4). Alicia Keys "It's on Again" - Music video.
5). "The Reward of Heroism": The making of "The Amazing Spider-Man 2: Rise of Electro".
6). Over 100 minutes of detailed featurettes about the creation of "The Amazing Spider-Man 2: Rise of Electro".
It may, therefore, be the case that the bonus Blu-ray disc, provided in the German Steelbook edition, actually contains additional material to that listed above, the details of which have yet to be announced.
According to the announcements so far, this bonus disc appears exclusive to the German, French and Dutch Steelbook releases.
Given that the German Steelbook is available from amazon.de for EUR 19.99 (£16), this would appear better value than the UK release, unless you particularly want the poster. I'd rather have the bonus disc.