[aneisf]

Sure, but your identity persists across all subreddits. If someone makes an insightful comment on a programming subreddit, for instance, and has a high karma score, you may be disappointed to discover that 90% of their previous submissions consists of cat pictures, jokes, in-depth cartography discussion, and porn.

I guess reddit just feels 'diluted' to me. It facilitates a broad range of discussion and does so fairly well, but I can't think of a subreddit that is actually the best place to discuss any particular topic. It's generally my second or third stop.

[Terr_]

Easy enough, conceptually at least: Allow people to see other users' activity/karma which is only within the current subreddit.

[jeffbr13]

Similarly, your rep on one StackExchange site doesn't follow you around to all the other sites, because it's not really relevant.

[Terr_]

> it's not really relevant

Well, it could be highly relevant, but the system isn't (yet?) designed to recognize when two sibling sites have significant overlap, which does occur.