In Vanilla and TBC, the raiding endgame was based around the idea of progression. A guild did Molten Core first, and after they moved on to Blackwing Lair, then AQ40, and finally Naxxramas. This was the pattern guilds and players followed regardless of when they started. A new guild or raider was expected to start in Molten Core and move their way upwards.
In contrast, Wrath raiding was based around a different idea: focus on the current tier. In general, at any given time the community--including newer raiders and guilds--focused on the most recent tier. When TotC was released, that was what everyone did. When ICC came out, everyone went for ICC. You could essentially solo your way, gear-wise, to the entry point for the most recent tier.
So which style was better?
To me, they both have strengths and weaknesses. Progression felt natural and organic. You moved through the raids and as you got better you did harder raids. You experienced each tier of content in roughly the same difficulty as it was intended. There was this sense of "being on the path" which really doesn't exist any more.
But on the other hand, guilds often got stuck on the path. There would come a boss or point which you just could not beat with your current guild, and that was that. And very few guilds actually got a chance to see the last couple tiers. The vast majority of guilds stalled out a lot earlier.
Secondly, I don't think recruitment was healthy in the Progression era. In order to sustain itself, a guild had to pick up people at the same level or just below to keep going. That meant that the high-end guilds poached the better players from the tier of guilds below them, and that tier in turn poached from below them. Players were always moving up from guild to guild, because they had to join a better guild in order to see newer content.
I know that in Wrath, a lot of the Royalty guilds have been complaining that recruiting has gotten harder, but I have no sympathy for them. Previously, they had their pick of good players, because joining a Royalty guild was the only way a player could see content like Naxxramas. Now, a good player doesn't have to leave her guild just to see new content.
Current Tier Focus Style
The Wrath model also has strengths and weaknesses. It's greatest strength was that a much larger percentage of the player base got to see the newest content. If you raid at all in Wrath, you've gotten to see ICC, and quite possibly have gotten to see the Lich King. Compare that with the percentage that saw Kil'jaeden or Kel'thuzad.
Second, I think it may have made recruitment, and bringing in alts, easier for lower tier guilds. They didn't lose people to the high end guilds as often, and newer recruits can solo to a reasonable entry point for raiding. We don't have to carry people through a lower tier to gear them up like we used to.
I really think that it has also made raiding guilds that target a shorter number of raid days or more casual atmosphere a lot more viable than they were in Vanilla/TBC.
However, the Current Tier Focus style does have downsides. First, the lower tiers of raid content got obsoleted. Very few people do Naxx and Ulduar now. And that means that raiding isn't quite the shared experience that it used to be. For instance, all raiders in Vanilla went through Molten Core. If I say "Loot the Hound", every single person who raided in Vanilla understands me. Whereas if I comment about the Heigan dance, there are players in ICC right now who won't know what I am talking about.
Or difficulty-wise, pretty much everyone who did BWL understands the challenge of Razorgore or Vael, because we all did it at more or less the same difficulty. But someone who gets taken to Ulduar now, in full i264 gear, just doesn't experience it in the same fashion as those of us who did it when it was the current focus.
The other major downside is that this model is very sensitive to the amount of time that an instance exists as the current tier. It is very arguable that the Wrath experience would have been a lot better if Ulduar had lasted for 2-3 more months, TotC 1 less month, and ICC maybe 1-2 less months. TotC was actually the current instance for longer than Ulduar.
In the Progression style an instance was "current" for as long as you were working on it, which is pretty much the ideal amount of time. You want just enough time to beat it, and maybe farm it a couple of times to finish off your set, before the new instance opens. Unfortunately, that amount of time is completely different from group to group.
So which style would you pick? As much as I liked the Progression style, I would much rather have the Current Focus style. The people at Elitist Jerks are very much in favor of the Progression style, but I think they suffer from survivor bias. It was a lot less fun for those of us in the lower tier to get stuck and lose players to the upper guilds.
Or worse, to have to choose to leave the guild and people that you liked because you knew that if you didn't, you would not see new content at all. For all the flaws of the Current Focus style, pretty much every guild that raids regularly can see all the content that Blizzard creates.