I'm a bit surprised this hasn't come up already really.

So, like RtL, the new props in SoB are not actually defined. Which props (if any) are obstacles?

Anchor
Barrel
Cage
Captains Wheel
Cavern Entrance
Forecastle
Hold
Mast
Pipe Organ
Railing
Reef
Rope
Sandbar
Scrub
Statue
Water (Deep)
Water (Shallow)
Weapon Mount (empty)
Weapon Mount (w Cannon)
Whirlpool

My Guess is that Anchor, Rope and Captains Wheel might not be obstacles since they have no discernable effects of obstacle type (they do not affect movement or have affects upon figures occupying them in any way at all).
Forecastle is likely just a prop since it too has no discernable effects of obstacle type, even though it counts as elevated terrain (a subtle difference).
Railing is unlikely to be an obstacle because it is not a space type - it fits between the spaces like a wall really.
Scrub might not be an obstacle because again, it has no effect on figures entering it or occupying it - its affect is on the attacks of figures not in the scrub itself (this is different from the shadowcloak given by trees, for example, because the trees give the shadowcoale to the figure in them rather than affecting the attacks of another figure directly).
The Cavern Entrance is probably not an obstacle because it is analogous to a portal space - similarly the Hold. The skull spaces may or may not be obstacles -in at least one island level they are impassable, though usually they have no effect.

Everything else (Barrel, Cage, Mast, Pipe Organ, Reef, Sandbar, Water (both), Weapon mount (both) and Whirlpool) seem likely to be obstacles.

The main reason for this coming up is the ability to ignore the effects of obstacles - fly, mostly (there is no acrobat), but also large monsters!
Thematically, probably only the cage is dubious as being 'ignorable' for flyers. Since it blocks movement*, I don't think it is an issue with large monsters (Mahkra's unease notwithstanding).

What do others think?
I'm thinking that this will need to go to FFG for an official answer, but I'm interested to hear what others think, or what they have done/ruled in their games.

*As an interesting aside, since a cage blocks movement can a (small) figure move around inside the cage? Perhaps the cage actually needs new rules entirely and 'blocks movement' is insufficient. Perhaps...
"Figures may not enter or exit the cage prop unless they fulfill scenario rules. Figure inside a cage may move around inside the cage freely. The cage is not an obstacle."
... which would solve all potential cage movement issues 'thematically', and reasonably, I think.

Which ones are obstacles hasn't come up yet, though your list looks good. We allowed movement inside the cage. IIRC you have to, as one of the quests starts everyone in the cage and only those next to the wall can escape.

The Sea of Blood rules p35 - 38 update a lot of obstacles and props including the cage. It notes that it includes the collected descriptions of all obstacles and props from Descent and all of its expansions to date.

The cage has been updated to "A figure cannot enter or leave a cage, although it may attack through the cage. Normally the quest that a cage appears in describes how it can be opened," a lot like you described.

James McMurray said:

Nope, you don't have to be next to the edge, you get placed to the edge (which is it's own problem if more than 2 heroes roll surges).

We've fought that level twice, with two different OLs and have scores of 30-0 and 21-5 in favour of the OL.
The first time the heroes were up against silver eldritch and couldn't get through the Golem while the 2 command from the boss Naga helped Skeletons and Shades butchers them. They pushed hard because they were 3rd level down trying to grad a plot token shard thing, but eventually bailed.
The second time was against just copper monsters but a combination of OL lucky attacks and hero misses, followed by around 20 non-surges in a row for getting out of the cage saw a melee hero, alone blocked by shades for about 6-8 turns, including going back to the cage and being handed a (purchased from town) Walking stick followed by 6 consecutive missed attacks with the walking stick. Eventually one of the mages managed to get out of the cage and survive one turn so she could fly through the crowd (OL screwed up by having other monsters behind the boss Naga that could be anti-grappled through) to the other Glyph. Once I went home (sick wife) the heroes were able to get through since the misses dried up... Heroes were pushing on this level because OL will upgrade to silver monsters at the start of next week and need cash and CT for at the end of this week.

The big plan coming into this level was to use the fly feats to get straight out of the cage, but the OL declared that the cage wasn't an obstacle so couldn't be ignored. Given the thematics of the cage, fair enough, but this got me thinking what could and could not be flown over...

mtnshredder718 said:

No offence meant mtnshredder, but all that is already known and utterly beside the point.
We known all these things are covered pg 35-38 (it's the only place they are defined because they are new in SoB) but they aren't actually defined as 'obstacles' or anything other than 'props' - and some of them undoubtedly are obstacles. Exactly the same thing happened in RtL and was later covered by FAQ. Props are a large group that has subsections including obstacles, traps, treasure and 'other props'.
Entering and leaving the cage is defined, though if the cage were an obstacle it could be ignored by flyers. However the cage is also defined as blocking movement, which has other implications.

We played it once, with me as Zyla. I don't remember if we let me fly through it or not, but it didn't matter much. Thanks to the nagas I couldn't actually get to a glyph easily to let everyone else escape. We took a beating on that level too (no idea what the final ratio was).