I was in a situation where I lost due to a combo where a creature that deals combat damage mills me due to an enchantment attached to it. I lost the game due to not knowing that ability was on the card. My opponent knew but decided not to tell me when he declared that attacker. Was it the defenders (my) job to read the card and block it, or was the attacker in the wrong for not declaring its ability when attacking. Also, if the trigger goes off as a result of the defending player not knowing, would the defending player be allowed to reassign a blocker in response?
It's your job to read the card, no matter what the situation.
Your opponent never has to explain cards to you. You can always request to read any card, and if you don't know for sure everything that a given card does, you probably want to read it. (If it's an old card or a foreign card, you could also look up the current oracle text of the card in Gatherer.) Your opponent has no more obligation to let you know that a creature has an ability than they do to remind you that it has enough power to kill you.
Assuming that creature had a triggered ability (like Mindscour Dragon), your opponent is responsible for remembering that trigger. But they only have to do so once it triggers and needs to affect the game state, i.e. once you've already let it hit you and it's going to mill you. They don't have to warn you ahead of time.
Your opponent is certainly allowed to help you out if they wish to; in casual play you might end up doing this in order to have more fun games and not waste time on really obvious mistakes. I've certainly pointed things out sometimes when it was clear my opponent didn't realize what my cards did, in friendly kitchen table games when they'd have done the same. But there's definitely no obligation to do this under the rules.
So bottom line, you just made a mistake, and you're not allowed to go back and change a decision because of it. It's unfortunate for you, but hopefully it provides some incentive to read the cards more carefully next time.
If you want to know more details about player communication, it's covered in the tournament rules - search for "player communication". If you actually ask your opponent a question, there are some things they do have to tell you (covered under "free information"). But they don't have to volunteer anything beyond what's necessary to progress the game (e.g. "I cast this", "I attack with this", "I take 3 damage", "that creature should be dead").