Henry said:
On the downside, it cuts out a wealth of adventure opportunities by doing so, and means that adventures will follow a similar format all the time, as you said the players will be reactive all the time instead of proactive.
How so? However you glean the information on a threat (through divination or spies), adventurers are almost always
reacting to a threat. I'm not sure how it would keep from being more proactive other than limiting the ease with which you can engage threats - and that's a very,
very good thing for the campaign IMO.
Take away the powerful divinations and you really help with taking away one the easy shortcuts through an adventure. Further, it keeps those "detective" skills valuable at high levels and makes it easier to design adventures. It's also easier to keep the PCs thinking and on their toes.
From a campaign perspective, I find that taking those two abilities away actually very
liberating. You can have (relatively) numerous high level beings in the campaign and not worry that the setting is going to break under their weight. They can't glean what is going on in the world on a whim and they can't get to trouble spots across the country with a 6 second spell. That keeps them focused on dealing with (primarily) with the big problems because that's all they'll have time to deal with. Many will be rooted to a city/domiain under their protection because should they leave it for an extended period, it could become a target to their enemies.
As a DM, you can handwave overland travel time if you want to avoid having to play it out, and you can use that portal the legion of Orcus worshippers were opening to springboard adventures into the Abyss or what-have-you. Personally, I see it as a win-win for the game.
To me, it's like saying that you can only conduct a war with rifles instead of missiles and bombs, or only using fire-and-forgets when you have smart missiles at your disposal. (By "disposal", I mean that the game used to have them, and now no longer does, effectively trying to put the toothpaste back into the tube.)
One of the largest complaints we see is the all-seeing, all-knowing plot device wizards who can just pop in to save the day any time they choose. Yeah, it's taking away toys they had in the past but I think it's well worth it - whether you simply retcon it as always being this way or have some FR-style event to alter how magic works.