Most of the EverQuest expansions have involved some progression tasks of quests in order to gain new items, spells, equipment, and access to dungeons or other zones. In many cases some of the older keys and flags are no longer needed. Some still are, however...

The Seeds of Destruction expansion gives you a chance to go back in time. You start out with access to Void(A). Completing the Oceangreen progression unlocks Void(B) and Journeyman Tier II mercenaries. Completing Kithicor unlocks Void(C) and Tier III, and so forth. Progression does not need to be done in order. At a minimum you will probably want to complete the Invasion of Rathe tasks to unlock the Journeyman Tier V mercenaries.

The Dragons of Norrath expansion has a nice progression of quests with a set of rewards that remain worthwhile even after several newer expansions because of the rewards: a total of 8 AA points and 5 new AA's that raise your stats, combat abilities, a level 69 spell-song-tome, and titles.