I want to take a second to discuss what I consider to be the cutoff between additive vs derivative gimmicks. This distinction goes beyond a simple question of taste and should not be seen as me telling you what to enjoy. Sometimes I may grade a derivative blog higher than an additive blog! The world is just another marble for me to play with. This distinction also only applies to gimmick rebloggers. Gimmick blogs that primarily make their own posts like "I-post-x-daily" are additive by default as they have nothing to derive value from.
The difference between these two is a little self-explanatory. An additive gimmick contributes to the original post in some form, whereas a derivative gimmick only extracts value created by somebody else.
The main problem with this definition is that a whether a gimmick is derivative or not often comes down to an individual post, but to judge the gimmick on a whole you must take into account the whole blog. Take for example the @silly-detector gimmick. I first came across clowns blog through the balanced humours post (see below).
one of each……. The Balanced Humors
This of course, is an exceptional post. The gimmick is additive, it clearly adds to the post and creates a new punchline. However, silly-detector also creates quite a bit of counter/detector-slop posts. In these posts, doing what amounts to rudimentary blackout poetry really doesn't add much to most of the posts in the same way that something like an identifier gimmick would, and thus they simply exploit the value of the original post.
I do say most for a reason, which brings me to what I see as one of the more important parts of a gimmick blog: how closely they stick to the theme. When silly-detector is detecting sillies in posts that center on mimes or clowns or the like, it is sticking to the theme of the gimmick. In that case, they are contributing to the post and continuing on with the bit, no harm no foul. Think of this a little like a conversation; you can add to the conversation by staying on topic, but it's usually rude to constantly shut down a conversation by changing the topic to something only you care about.
This leads what I consider to be the best litmus test for judging a gimmick blog: is there a person in my life that I would like to recommend the blog to?
A blog that is dedicated enough to it's theme may be derivative of individual posts, but can still be additive to tumblr more broadly by providing a collection of themed posts. I would recommend "x-heritage-posts" to somebody interested in "x".
A blog that is additive more generally is something I would probably recommend to anyone with a similar taste to me. Think blogs like the universally beloved haiku bot.