Strahd being too kind
Reading various posts on this subreddit shows that issues arise when DMs portray Strahd as too kind or gentlemanly. When players do not feel constant danger, they tend to:
(1) Ignore the main quest to complete every side quest.
(2) Openly mock or underestimate Strahd.
Despite his gentlemanly demeanor, Strahd should always remain sinister, evil, and oppressive.
Another DM trap is treating Curse of Strahd purely as a sandbox, not a quasi/semi-sandbox. While CoS offers open exploration, a subtle linear narrative runs beneath it. Running the campaign too loosely causes players to lose direction. Players should be constantly (1) pushed by threats or (2) pulled by NPCs, toward certain directions, even though players have options on where to go.