TBC content is definitely a yes, but TBC also took from us the great dungeons that were clearly inspired by sebilis, guk and so on and instead gave us the modern military shooter type levels of short tubes, exactly three bosses and one final boss and that's it. Trash became exactly that - trash. And of course now the levels are designed in a way that you can't skip any of it really. You would want to, because trash doesn't drop anything anymore (random blue BOEs like in Shadowfang, or GFPP recipe in LBRS?)
to me TBC is where they started to design the vanilla content already. Nothing out of the ordinary, nothing special. Just produce content for the players to consume. No details, thanks.
Also flying mounts started this massive zoo that retail is today, where every city is full of the most absurd, massive flying things coming in all kinds of color, even LGBTQ parades pale compare to what a citywith 30 mounted players look like these day. It's a stupid circus.