WotLK introduced a great deal of concepts that softened the Classic feel–at the time, many of those changes were popular, but over time they’ve become recognized as a major contributor to the game’s transformation from Classic to Modern. Here’s a short list of some of those things:
1.) Raid difficulties. It started with Naxxramas, giving guilds the option to do either 10 man or 25 man versions of the same content. By the time Icecrown came out, that was extended further to there being a “Heroic” difficulty mode in addition to the standard. It divided the content into multiple forms with different mechanics. Some people view this as having “watered down” the experience of raiding, making it simultaneously more accessible but less meaningful.
2.) Dungeon Finder. This one is a gigantic contributor. It completely removed the need to socialize in order to find and/or keep new group members if/when one leaves, or when starting the group to begin with. It turned dungeons from an event to a treadmill–no longer did you organize and plan, then travel to your destination. Dungeon Finder threw you into a queue, automatically got you a tank, healer, and 3 DPS, then teleported you directly to the dungeon. If somebody left, they were automatically replaced within minutes. It made the game, again, more accessible but less meaningful.
3.) Badges of Justice/Triumph/Frost/etc. These existed to some extent in Burning Crusade, but they were only available from Heroics. WotLK softened the difficulty of Heroics without removing the potency of the gear rewards. Furthermore, as new raid content released, the badges became “stronger” and were capable of purchasing raid-level gear as a reward for completing early-expansion content that hadn’t gotten any harder. It took time to earn enough badges, but it did not take anything more than doing heroics.
4.) Achievements. Classic (and TBC) were about discovering and earning new rewards for the sake of having the rewards. WotLK took that a step further and assigned achievements to those rewards as well. Now, players were given a literal book of things that could be done for a “reward” of 10 achievement points and an entry in your achievement journal. No longer was World of Warcraft a free-form adventure where you pursued the things that mattered to you–with the addition of achievements, it became a glorified to-do list, literally checking off boxes as you churned through content arbitrarily deemed “achievement worthy.” It added longevity to the game, but damaged its soul.
5.) Death Knights. While I love Death Knights and I don’t think it’s a bad thing to add new classes to an MMO, it was the first time Blizzard experimented with “hero classes.” Death Knights were ridiculously powerful at first. They started at 55 (58 by the end of the intro), every spec could either tank or DPS, and (at least in my guild) they obliterated the DPS charts (often by literally using the ability “Obliterate”.) Like I said, it’s not a bad thing, but ever since DKs were added, the concept of a powerful fighter/self-healing class has permeated the game’s design. Almost every class in BfA has some kind of similarity to the playstyle of WotLK Death Knights.
6.) Profession Changes. These are smaller, but significant. For example, from patch 3.0.8: "Mining veins and deposits no longer require multiple hits to receive all the ore. Players will receive around the same amount of ore, stone, and gems they would have received from multiple hits. " Basically, the entire gathering process was streamlined. No longer did you pick up ore one at a time, but as a big bunch with one cast time. Did it make the game “better?” Yes, it did. But it changed it from the slow pace of classic to the “get as much as possible as quickly as possible” atmosphere we see today. Nowadays, there isn’t even competition for ore/herb nodes–everybody gets their own version of each spawn, so there is a practically limitless supply of gathering materials out in the world. Back in Classic, such things were restricted by spawn rates.
There are more things, surely ones I’m forgetting, but that’s a basic run-down of what I think most impacted the game to turn it from “Classic” into “Modern,” or “what it is now.”
I don’t dislike modern. I still play it. I’m logged into my Demon Hunter in another window in the background right now, as I type this. I just miss some of the experiences of Vanilla, and I want to go back to that.