Two things.
1. Voting: It changes the incentives for posting a comment. It becomes a popularity contest where people go after easy points by posting low quality but popular comments, rather than focusing on quality discussion. Similarly, quality posts are suppressed if they are unpopular. This behavior is killing what's left of this site too, but that's a discussion for another time.
Ordering posts chronologically removes this manipulation. Real life discussions are not threaded, and aren't voted upon.
2. Forums required investment by parties sufficiently dedicated to a topic: you need hosting, administration; this costs money. Reddit on the other hand inverts the equation. It incentivizes the worst of the worst people on the Internet (penniless losers with unlimited free time) to usurp power and build fiefdoms. In a way Reddit exploits them because they do their dirty work for free. This is who ends up controlling subreddits.