Hi the_bork, I agree with you on the fact that it may be a feature and not a limitation; I think a possible feature would be to specify which galleries/albums to look for tags in.
I think that what confused me at first is that I found the shortcode for [nggtags] confusing when compared to the rest of the NGG shortcodes:
[ nggtags gallery=<tag> ]
[ nggtags album=<tag> ]
While for the gallery and album shortcodes:
[ gallery id=<gallery_id #> ]
[ album id=<album_id #> ]
I think it would be clearer if the shortcode would be something like:
[ nggtags tag=<tag> ]
This could be extended to:
[ nggtags tag=<tag> gallery=<gallery_id#> album=<album_id#>]
I guess the concern in this case would be that it would break previous installations, but I guess a workaround would be to have a script that would convert the shortcodes in the post/pages upon the first activation.
Just a thought.