1) Sort of directory: http://www.reddit.com/reddits/
2) Usually, there are links in subreddits descriptions to similar subreddits.
2.1) MultiReddit! http://www.reddit.com/r/php+perl+python+startups
3) It should be improved, yes
4) I guess cross-post and double-post are hard, really hard to improve
5) search is a bit awful, yes