These two missions have some of the best production values I have seen in Thief. Ominous Bequest's epic mansion exploration is only rivalled by Rowena's Curse, and Broken Triad has all those little elements and epic moments that question it can really be done in this engine/game. Waking up in a mortuary and getting out with your skin was one of those frightening sequences which are hard to achieve now after having played Thief for 12 years, but it works perfectly.
On the other hand, I think BT's genius is in the first mission, and the second, while neat, loses some of the interest and tension. It is more like a "what happens after the main conflict" thing; important to finish the various plot threads, but definitely post-climactic, and too long for that.
And yes, I missed that hidden area (although I found the Horn of Quintus, my favourite WTF moment).