If I have a Bloodcrazed Hoplite that's been given +1/+1 counters from its heroic ability, and then it's killed by an enemy creature, and then I cast a spell that returns it to the battlefield, does it still have the +1/+1 counters on it?
No, it does not keep the +1/+1 counters. Specifically, rule 121.2 says
Counters on an object are not retained if that object moves from one zone to another. The counters are not "removed"; they simply cease to exist. See rule 400.7.
and rule 400.1 says
A zone is a place where objects can be during a game. There are normally seven zones: library, hand, battlefield, graveyard, stack, exile, and command. Some older cards also use the ante zone. Each player has his or her own library, hand, and graveyard. The other zones are shared by all players.
So, when the Hoplite dies, it moves from the battlefield to the graveyard, and its counters disappear.
More generally, rule 400.7 says
An object that moves from one zone to another becomes a new object with no memory of, or relation to, its previous existence.
It has exceptions, but none apply here. The Hoplite that you return to the battlefield counts as a completely new creature with no memory of its previous existence on the battlefield.