In practice, the word "troll" has come to replace "dissenter" and "devil's advocate" and other terms with good connotations. It does encourage groupthink--as soon as someone differs from the party line, bang, that person gets labeled as a troll, end of discussion. This is very common on political sites (viz, sites that have party lines).

A real troll starts out making small, reasonable claims, and then, after getting people to agree with him at first, makes larger and larger claims. The precedent is "A modest proposal".