On the one hand, this is fine: Reddit is supposed to be a collection of independently moderated sub-communities with their own rules and administration. On the other hand, you have a unified identity and content history across those communities, so it's a lot easier for one community to take action based on your history in another, which is a strange dynamic.
I actually think Facebook Groups are onto something with the way post history and profiles work: each Facebook Group a user posts in creates a separate sub-profile for that user which is specific to the Group. Users in that Group can see a user's post history in that Group, and that user's "main" profile depending on their privacy settings, but a user can't walk "across" to see a user's post history in other Groups unless they search from that other Group.
I feel like per-subreddit post histories along with a global user profile would help move Reddit more towards the "sub-community" vision if that's the direction they want to go.
The issues Reddit have are:
* Cross-stalking, as discussed above.
* Content discovery. This is the same problem every user-generated content platform has. What sub-communities get surfaced on the logged-out front page? Cross-pollinated to existing users? Every type of content will be objectionable to someone, so deciding what to show is always going to be a lightning-rod issue with advertiser dollars at stake.
* Global moderation. What's "bad" enough to get a user banned from _all_ of Reddit? What happens when that user is completely banned (do all of their old posts disappear?) Should large-scale content moderation like spam be handled at a platform or a community level?