When Lotleth Troll regenerates, do the +1/+1 counters stay on it or do they go away.
Same thing with Exile. When Lotleth Troll gets O-ringed or detention sphered, do the counters remain on him when he comes back?
Regenerate will have no effect on the counters on the creature. It simply removes the damage dealt to it, taps it, and removes it from combat. If your creature is exiled, it will come back to the field as if it were just put on the field in the first place; the counters will be gone.
RESOLVED
Counters are removed when zones change. With regeneration, lotleth never leaves the battlefield, and will keep his counters. With o-ring, he changes from battefield to exile, and the change of zones will remove his counters, as well as enchantments and equipments attached to him.
{skullbriar, the walking grave} is an exception to the counters ruling, if youre interested.