In the first Divinity - Original Sin, Battering Ram did not trigger attacks of opportunity. I just used this ability a few minutes ago, and took hits of opportunity from each enemy I passed. Is this intentional? I see nothing in the skill description that says it should be like this, and if it is supposed to be, it really takes away from the skill's value, considering you would only ever use it on one target or if you didn't care about taking the extra damage.

I've had this happen as well.
This doesn't happen for me most of the time. Perhaps it only occurs when it fails to knock the struck enemies down.
Also, many enemies do not have opportunity attacks.
The ability is still very strong for 1 AP. More damage than a regular attack, and at least as cheap, if not cheaper.

In the Deep-Dweller ambush one of the skeleton warriors even used Battering Ram against somebody with high-advantage, making him run through the air. He fell afterwards back to the ground because he could not get a foot on the high ground.

With attacks of Opportunity we had never problems, mainly because we only use it, if we know for sure, we get them knocked down, tough. But attacks of oppurtunity if you approach an enemie is a new add or a bug, not sure. Can be pretty annoying.

This also happens with Cloak and Dagger, Tactical Retreat and Phoenix Dive. If used when next to someone with Opportunist, they'll trigger an opportunity attack. I've died this way several times.

Odd, I never noticed it with those teleport-style spells. In fact, I often used them specifically to avoid this.
Does it occur for you often or just rarely with these spells?

Rarely with C&D, often with Phoenix Dive, and Literally. Every. Time. with Tactical Retreat. I have died exactly three times by tactically retreating out of opera range. I can recall two fights specifically;

Yes, this is a bug, and a known issue.