Long, rambling post follows
I felt that the published
Dragon Mountain promised much but failed to deliver. When I ran it, I modified the adventure massively. I ditched the entirety of the first book, replacing it with quests more relevant to my homebrew campaign, although I kept the idea of gathering together three pieces of an artifact needed to enter the Mountain.
In my game, the Mountain was inhabited by several dragons, all descendants of an ancient Wyrmsage. The Wyrmsage had visited my gameworld milennia past, shedding three tears on her departure. These three tears became the three pieces of the amulet around which the opening quests are based.
In the interim before the Mountain's return, the Wyrmsage had become possessed by an alien entity and had locked herself in her inner sanctum within the Mountain. I replaced the standard cave lair from the published adventure with a massive, 5-mile wide egg-shaped chamber at the heart of the Mountain. Her brood (which included both chromatic and metallic dragons) fell to rivalry deep within the Mountain, each vying to replace the now-inaccessible Wyrmsage.
I removed the kobolds entirely and replaced them with clans of draconic humanoids. Debased offspring of the Mountain's dragons, these draconic humanoids served the dragons as slaves and vassals and inhabited the majority of the Mountain, leaving the deeper regions to the dragons themselves. The draconic humanoids had a messianic prophecy that foretold a time when they would start to develop metallic or chromatic traits (formerly, they were just nondescript, scaly beasties who revered the iconic natures of the dragons from which they had descended). The appearance of the first iconic draconian would also signal their release from bondage to their dragon masters.
The draconic humanoids were divided into clans (loosely based around the kobold clans of the original adventure), the most powerful clan being in control of the area directly adjacent to the deeper regions inhabited by the Mountain's dragons. The clans were at war with each other, some still loyal to the dragons, others not. The temple on the lower level was occupied by a philosophically minded clan that acted as keepers of the messianic prophecy (viewed as a dangerous heresy by the dragons and their loyal followers).
As the Mountain had travelled to many different worlds, I also devised a settlement that had sprung up on its slopes, inhabited by outcast draconians and a mish-mash of creatures from various worlds who had tried to gain entry into the Mountain, but had been refused for one reason or another. They had instead settled on the Mountain's slopes and travelled with it as it shifted between the planes. This "City on the Mountain" occupied the PCs in my game for a couple of sessions before they actually entered the Mountain proper.
The appearance of the Mountain in my game was linked to a magical condition that was killing off all the dragons of the world. It signalled the last chance for dragons to hitch a ride to safety, as well as a way for PCs to consult the Wyrmsage and answer other campaign-related dilemmas connected to the unavoidable death of all dragons and, ultimately, the end of the gameworld itself.
Local dragons (including a long-running dracolich nemesis of the PCs) from my gameworld had raised armies and were beseiging the City on the Mountain when my PCs arrived. The PCs allied themselves with the City and fought in the seige. They used the Dragontear amulet to enter the Mountain and became embroiled in the conflicts between the clans of draconians and the heretical messianic prophecy.
Ultimately, the dracolich and his forces were able to penetrate the Mountain and attacked the citadel of the largest draconian clan in the deepest parts of the Mountain that bordered the region claimed by the dragons. The PCs were instrumental in defeating this assault and (unsurprisingly) in discovering the identity of the prophesied messiah (a draconian born with golden scales).
However, upon revealing his identity to the dragons, the messiah-figure was promptly eaten. While the PCs battled their way through to the inner sanctum, playing the rival dragons against each other, the draconian clans began an outraged uprising, ultimately leading to their freedom and the fulfillment of the prophecy. The PCs were able to reach the dying Wyrmsage and assisted one of her offspring in taking her place.
Anyway, those are the bones of it. I felt that, for an epic adventure, the published version fell rather flat, so I added the above elements to give it that extra oomph. It ran for nine sessions of about 10 hours each. I also played in the adventure as a PC under a different DM - he ran it more or less out of the box, which was enjoyable enough, but proved a bit of an anti-claimax once we finally reached the Mountain itself. He also changed the ending of the adventure to meet the needs of his own game (including the much-applauded death of Elminster, wahoo!) and I'd recommend that you tweak it heavily yourself. It's worth getting hold of, just for the setting and the maps, but it needs customising to give it the impact that it deserves.