Spend more RP time in Diamond Lake. There is a lot of potential material to be developed over the course of "The Whispering Cairn," "The Three Faces of Evil," and "Encounter at Blackwall Keep" that can be used as strong tie-ins to "The Hall of Harsh Reflections," "The Champions Belt," "A Gathering of Winds," and "The Prince of Redhand" (and possibly even "The Spire of Long Shadows).
As a player - nothing, I had a blast. Well, maybe prepare better for Darl and his gang, as they TPKed us.
As a DM - I allowed for more flexibility and opportunities in Diamond Lake and the Free City of Greyhawk. For instance, I included Balabar Smenks Mansion (found here on the boards), am running the Return of the Eight sidetrek, allowed for much more interaction with Auric's Warband (They didn't get to like each other...), and so on. More freedom of choice, less sure ground and railroading.
But most importantly for the feel of the adventure path, I built upon my players initial mistrust of Allustan, and kept him on the shady side of Diamond Lake. Instead, Filge pointed them to Eligos (who had a much more sinister streak as well), they had to overcome their mistrust of him. Celeste is a more questionable personality as well, always busy with unknown agendas and pulling strings in the background.
All in all, they don't yet have an ally they can fully trust. They'll earn one once they restore Tenser from hist last "dretched" clone.
Limit the access to splat books. While it was fine at the lower levels, at the higher levels I am finding the party outstrips the monsters to the point where they can handle anything that I throw at them, leaving me with a lot more work to scale things upwards. Many things don't take into account spells like the Orb spells, which bypass SR, making what should be a very difficult encounter not so tough.
I'd also consider limiting access to the leadership feat and companions. In the end, the core party was 4 players, two with leadership (so two characters per player), a druid (with 4 animals, each animal with 4 attacks), and a ranger with an animal companion= a party of 11 (plus some other help summoned to their side). So you can see where combats take forever with many combatants. I took a picture of the party fighting in the Citadel of the Weeping Dragon, right after they got the main objective there. I threw Brazzamel and Gorundul at the same time at the party and they lived, only losing one in the party.
We are just starting into Wormcrawl, and the party is level 19. I will very likely just grab the essential story elements, plus the return of some bad guys who got away earlier (whom I will beef up), but pretty much more than half of this current adventure won't be needed nor seen.
Next on my DMing queue is Savage Tide. That I hope to run in Pathfinder, and it will be restricted to Pathfinder classes only. I'll have 6 months to work on that while the other DM takes the helm for a bit.
Two and a half years we will have been playing Age of Worms biweekly, and it's been a blast too! Thanks to all the writers... Whispering Cairn still sticks in my mind as one of my most favorite adventures in 3.5 edition.
If I could change anything, it would be the Adventure Hook for that opening adventure. The PCs are looking for treasure so that they can leave the town of Diamond Lake and head for Free City. Fair enough. However, when our group got to Alaster's house and found that the bodies of his parents had been taken back to the town we were all trying to get away from, our party started debating about whether we should just sell the lanterns we found in the Whispering Cairn and cut our losses.
Personally, I liked what one of my other DMs did. A wizard friend sent the party an invitation to come visit him at his home outside of Diamond Lake. When we got there, we found he had been assassinated by members of the Ebon Triad. After defeating the assassins, we found notes from him talking about the Wind Dukes of Aaqa and the Whispering Cairn. Thinking that we might discover the motive of his assassins if we went there, we decided to investigate the lead.