Combined with the bandwagon effect of downvotes in many (not all) subreddits, this has the same filter-bubble effect, just by virtue of pushing differently-minded people out (either to seek other subreddits, or off the site altogether).
Just look at /r/politics and you'll see this in full force - with a few exceptions, almost any post expressing a somewhat conservative perspective is downvoted massively.
This is one of the reason Reddit implemented [score hidden], which seems to have helped somewhat, but not entirely.
> Radicals tend not to like people disagreeing with them.
This isn't true only "radicals" - it's true everywhere. And I don't think it's the userbase that's different; it's the structure around it that makes the difference. In other words, the same people will group themselves differently on Tumblr and Reddit, even if they use both sites.
I think it's the bubble-ish environment that gets created not only by your followership but also by the fact that tumblr blogs rarely get feedback from outside.
Once you've build a sufficient followership around you, every thought you express in that bubble gets some how discussed, accepted and developed.
That's why for once tumblr blogs react so harsh to bubble-out-side topics (/r/shittumblrsays) and secondly why I believe it is such a good place for discussion of "radical subcultures".