Burning Crusade launched in January 2007, and from that point until November 2008, level 70 was the endgame of World of Warcraft. For roughly 23 months (with staggered content releases, with the Black Temple launching later, then Zul'Aman and then Sunwell/Magister's) we all leveled to 70, ran heroic instances and Karazhan to gear up, and then some of us began making our way through Gruul's Lair, Magtheridon's Lair, Serpentshrine Cavern and Tempest Keep before moving on to Mount Hyjal and the Black Temple.
Zul'Aman came out to offer scaled challenges (the prototype of the hard mode) with a timed run to get the Amani War Bear, and of course no one can forget the final big content patch, Fury of the Sunwell, which gave some content for just about any level of gameplay from casual daily quest grinding to hardcore raiding. Gameplay was still very stratified in Burning Crusade -- there were a handful of guilds progressing through the endgame content (which was still tiered into a couple of 10-man raids, with the majority being 25-man) but most people did dailies, ran some BGs or played Arenas (which debuted with Burning Crusade as well).
With Cataclysm previews coming out and Wrath in its last major content phase (Icecrown Citadel and the upcoming Chamber of Aspects raid being pretty much the end of Wrath's end game) we can start to look back at how the last expansion unfolded and the life cycle of an expansion in World of Warcraft.
Badges of Justice originally dropped in heroic dungeons as a way for people running those instances to be sure they'd eventually be able to pick up something. Eventually this badge system was extended to raids, and the rewards available were on par with raid gear to the point where someone who ran nothing but heroics and 10-man raids could get weapons and armor as good as someone running Sunwell. This led to people farming heroics and the 10-man raids as much as they could in order to gear up for harder content ... a prototype for the current dungeon finder system. In addition, the popularity of 10-man raids (especially Karazhan) led to the introduction of variable 10- and 25-man raids in Wrath, where the same bosses could be encountered tuned for groups of different sizes.
Of course, this all changed with the release of Wrath of the Lich King. In fact, it all changed with the release of Patch 3.0 before Wrath itself launched.
With the release of patch 3.0.2, player power levels went up with access to 51 point talents, while most raid bosses saw a decrease in their health and damage output. Essentially, everything was easier at the exact same time that everyone got stronger. As a result, raids blasted through content they'd been stuck on before (helped in no small part by gear purchased exclusively through Badge of Justice vendors). It's interesting to look back at the very end of Burning Crusade and see how many changes and innovations in Wrath are directly learned from the progression from very early BC to the end of its life cycle. The initial tiered Emblem of Heroism/Emblem of Valor system in Wrath was a direct response to complaints that badges in BC let people gear up without experiencing the content, while the constant release of progressed 5-man instances in Wrath (first Trial of the Champion and later the three ICC 5-mans) and staggered Emblem of Conquest, Triumph and then Frost rewards allows for exactly that: a means to catch up on gear as new tiers of raiding are released.
Unlike in BC, where you could effectively get weapons and armor near to the absolute top of the line through the badge system simply by running enough heroics and 10-mans, the current emblem system allows one to solidly gear up to begin the top tiers of raiding, but does not replace them. It provides more general depth while still leaving significant holes that need to be filled through other content.
This is just one example of how a BC mechanic developed further in WotLK. The eternal see-saw of arena development and balancing is another example. Throughout BC, arenas and PvP were seen as a means to get good gear without raiding, gear that in some cases was as good as or superior to items raiding had to offer. While this was hardly the case by the time season 3 or 4 had come and gone, the legacy of seasons 1 and 2 caused arena play in Wrath to be balanced in the opposite direction. Many people walked into arenas in wrath with Naxx-10 or -25 weapons and several pieces of raid gear, and as a result, we saw buffs to resilience aimed at making PvP gear attractive in PvP again.
The reason this is interesting as we head into Cataclysm is, for once, we're going to see the lessons learned in previous expansion development applied retroactively to the 1 to 60 game as well as everything above it. The various class previews all showed that old mechanics are being re-evaluated based on what has been learned in both BC and Wrath, and we know that mechanics developed in expansion like flying mounts and the way PvP has developed through arenas will be applied to the original content. This is the first expansion for World of Warcraft that expands what has gone before rather than just expanding on it.