Question though, is there quest lines or gates to get into content from expansion packs? Or is it all just out there?
The only barrier to entering a new expansion is the level requirement.
Wrath wasn't a bad expansion. Literally never met a single person of that opinion.
There was a lot of bad stuff about Wrath. For me it probably outweighed the good. Death Knights were broken beyond belief for way too long, and even after them the class balance never really got to a good place. Inscription was a total bore and had to be walked back and almost pushed out of the game. We got a one two punch of the worst battlegrounds that would ever plague the game in Strand of the Ancients and Isle of Conquest, and the Orgrimmar Arena was so bad that it holds a place in history as the only arena map to be completely excised.
Even using dungeon hubs to reuse assets the offerings were pretty weak. The Nexus and Azjol-Nerub stand out as winners, but if someone was making a list of the top ten dungeons in the game, I'm not sure Wrath would have a significant presence at all. If anything it'd be the foundation for a list of the worst. And the raid scene...was anything less inspiring than hitting cap to see Naxxramas again in a form that had defanged the most intimidating area World of Warcraft had ever known? For as impressive as Ulduar was and continues to be, its time in the spotlight lasted a paltry four months. Crusaders' Coliseum wound up standing as an equally important pillar to Wrath's raid scene, and it should not be remembered fondly in any respect. Icecrown Citadel ended the expansion with a whimper, though it helps that Arthas wound up being a really well designed encounter.
It is usually pretty disheartening to see people retroactively hold up Wrath of the Lich King as some kind of golden era for the game.