I've seen parts of the site fail to update in sync or appear to revert from time to time, so something like race conditions or cache invalidation does appear to be an issue. Adding another complex graph with a tag system might be a serious drag on performance, with everything being limited to file read/write speeds and there being no separation between data and application AFAIK.
Of course, the answer for that is for HN to bite the bullet and actually integrate a relational database, if only to improve performance and scalability.
And as far as lobste.rs goes, the culture there seems to be more restrictive due to being invite only, and having a policy of banning not only offending users, but the users who invited them, and the users who invited them, as far up the (publicly viewable) graph as deemed necessary. This inevitably leads to far more posts than comments, and less diversity of thought or critical discussion, by design. Nevertheless, using tags doesn't seem to have interfered with any of that, so it shouldn't necessarily interfere with Hacker News' culture either.
HN, however, has open membership, and any tag system employed here would need to take the heterogenous nature of the community into account. Since politics is one of the more divisive topics here, a politics tag would probably be inevitable. Lobste.rs would just not allow anyone they consider too political to join in the first place.