Ruhfuss
Savant
We might need a way to measure "openness" of a game to actually compare it.
It's a subjective judgement of course, but just for laughs you could use the number of possible game orders--for example Curse with its 3 main quests can be played in any of 6 ways (6=3!), whereas DQK is basically a railroad from what I remember. Pool had about 20 quests, which gives you 2 quintillion ways to finish the game.
Some orders are much more difficult though--the CRPG addict tried to finish Zhentil Keep first and you remember how that went. Of course one of the things unique about Pool was that you could actually finish the quests in different ways--you could talk to the old lizardman and fight a solo duel or go down the stairs and exterminate the lizardmen (and get their shields +2), you could fight nomads, kobolds, or both, you could bash open the box and get Cadorna pissed at you, be a goody two-shoes and hand it over, or get the thieves to rob it for you, and so on. I guess if you really wanted to you could multiply those in.
Hmm, I guess I was thinking more in a macro way (freedom in order to take on quests), and neglected the micro (role playing choices). Since I never completed the original Pool, but only the FRUA conversion once, which I'm sure isn't 100% on par due to FRUA's limitations, I can't comment on the quality of the "RPG-yness" of it. In most of the later installments, the RPG choices usually boil down to picking the most obvious (non-stupid) multiple-choice option ("You see a pit with many spikes at the bottom. Do you a) jump in? b) look for a way around it? c) [OPTIONAL|REQUIRED] use the McGuffin that you've found recently?"). Pools also offers some not so obvious stuff, that therefore can be easily overlooked, be it regocnition of player accomplishments for flavor like in the Arcam example above, or things like the different ways of dealing with the Gelt in Zhentil Keep. But nothing of this is a game changer, because it doesn't really matter.
Your Zhentil Keep example is a funny one, since if you recall some previous postings from different users in this thread, for Pools, going to Zhentil Keep too early often seems to result in as much pain as the crpgaddict might have felt during his Curse playthrough. I consider this to be one of the many homages to the precursing (PUN!) titles, although a less obvious one.