there are some key places i would like to go which no longer exist in their original form. 2 examples are the entrance to CoT, and the Dark portal being blue, and not green.
Also,
i took a break from the game from about october to may, and heard that silithus changed in P4. how did it change, and does anyone have any youtube links showing that change? (as i will not be able to see it myself anymore)
where else should i visit now that will change in P5 or P6?
now that im finally 60, where else should i visit in general that changed in tbc, wotlk or cata? first on my list is probably the Ghostlands area above EPL.
Silithus was emptier, that’s it. Less quests; I think it’s the same amount of NPCs as before.
I think you can wallclimb behind AQ gates before they open.
Dustwallow Marsh got updated in TBC. Mostly just additions. I think at the moment the zeppelin crash place is empty, while in TBC a questgiver appears. Not quite certain what lore says about that questgiver, where he is at the moment (or maybe I’m mistaken and he’s there near the crash).
Not sure about WotLK, but everything changed in Cata : D
There were little changes to the old world between Classic and WoTLK, off the top of my head…
WPL: a flight path is added next to the Thondroril river.
EPL: in WoTLK, Naxxramas is moved from Plaguewood to Dragonblight in Northrend.
Alterac: in WoTLK Dalaran is moved to Crystalsong Forest and the dome disappears, leaving a smoldering hole.
Swamp of Sorrows: NPCs with the name “Draenei” (like Draenei Exile) are changed into “Broken” (like Broken Exile).
Deadwind Pass: Karazhan opens in TBC.
Blasted Lands: the Dark Portal turns green. The area is cleared of elites and a few faction NPCs were added instead (of special relevance, the ones that start the Outlands quest chains).
STV: flight path added to Rebel Camp.
Dustwallow Marsh: as Nimerul said, in TBC Mudsprocket was added and with it a lot of new quests and a neutral flight master.
Ashenvale: in TBC all NPCs from The Shrine of Aessina are moved to Forest Song which becomes a new quest hub with an Alliance flight master and a few Draenei added to it.
Silithus: tons of NPCs and quests are added as part of the AQ event. (Might be too late for this).
Tanaris: the Caverns of Time open in TBC. Then, more dungeons are added to it in WoTLK.
Felwood: a neutral flight master was added to the Emerald Sanctuary.
During TBC most outdoor elite areas turn into regular areas (for example, Dun Garok).
In WoTLK tons of Spirit Healers are added.
If I understand correctly, by “the Ghostlands area above EPL” you mean the night elf ruins, which remain there until Cataclysm. In Cataclysm practically everything in the map changes; there are too many changes to be enumerated.