Smaller communities. You generally knew what kind of community you were participating in. Focused discussion. A forum was usually about a specific subject, rather than one account participating in dozens of completely unrelated communities.
Your identity was more compartmentalized. What you said on a linux forum didn't necessarily follow you into a motorcycle community.
Moderation had different incentives. If you ran a forum, you needed people to actually want to come back. Moderation that became too heavy-handed could kill the community you were trying to maintain.
Reddit essentially turned the forum model into one enormous site with thousands of sections, sounds great but broke more than it fixed.
Your Reddit account becomes a single identity spanning everything you do. Someone can look through your activity in completely unrelated subreddits, form an opinion about you based on that, and potentially use something you said somewhere else against you... no matter what the culture/norms/rules were of the other community.
A traditional forum depended on its community surviving. On Reddit, a subreddit can become extremely valuable simply because it occupies a desirable name and has accumulated a large user base. The underlying platform is doing most of the work of bringing people there. The moderator can be "all powerful" and heavy handed because they know rarely do competing subreddits actually work out.