Honestly, healing warrior tanks USED TO BE hard on healers. But honestly, this has changed a lot since Ulduar first came out. Our MT is a warrior, and I've healed him through nearly every encounter (and hard mode) in WOTLK. While he's not the best tank on every encounter, he's good enough on all of them, which is a huge step forward from when 3.1 first came out.
But one thing that caught my attention was the comment about "hard on healers" meaning healers going oom. This is not the only sign to be looking for. Another big sign is how much healers have to concentrate to keep a tank up. When it becomes "pure stress" healing a tank, then the healers are clearly completely overwhelmed. It happens. Even without healers going oom. It's just one of those things that you can't quantify, and i suppose some healers' threshold for "pure stress" is higher than others. But it can't be ignored.