Is there a way to start a private conversation with a player on the forums? I would love to be able to directly message someone to just strike up a friendly chat, exchange battletags or to prevent a topic from going off-topic.
This won’t happen because it would require Blizzard to hire more people to deal with PM toxicity, which in turn would take away from Bobby Kotick’s yacht money.
If they’re online you could /w them.
If they’re not currently online you could send them a Mail using the in-game mailing system. You can type words in there, not just send items or gold to your alts…