Is the DM obligated to tell the players that monsters are minions? I never do. Players don’t treat minions like they do other monsters. They don’t see them as threatening. And why should they, after all a minion only has 1 hit point and will fall with any hit that inflicts damage. In my mind the minion was one of the best additions to 4e D&D.
DMs can throw lots of monsters at the heroes, and as long as a bunch of them are minions then the PCs will just mow through them. They still have to target and hit them, but if a hit is scored the monster falls. There are, of course, some variations and exceptions to traditional minions – some minions explode when killed, some get a final attack and others still will rise from the dead to fight again after they’ve been killed once. Here at Dungeon’s Master we’ve introduced two-hit minions that, as the name implies, need to be hit twice to finally kill them. Regardless of what kind of minion you’re facing the commonality is that it doesn’t make a difference if you deal 1 point of damage or 100 points of damage with a single hit, you just have to hit them.
Minions are great and I encourage DMs to use them regularly, but DMs need to be careful how and when they reveal that some monsters are minions. As soon as player know that some for their opponents are minions they use this meta-knowledge to influence their PCs’ actions. For this reason I never disclose the fact that monsters are minions. I let the players discover this fact when their characters do. And if they make some poor assumptions I rarely correct them.