The Taken King brought with it:
Entirely new area, art and all
New world content in that area
New campaign missions
Three new subclasses
New enemy "race"
Four new strikes
Three new pvp competitive modes
Eight new pvp maps
New raid
And Year Two, which brought with it:
Year One content improvements
A new leveling system
Bounty improvements
Quest improvements
New social features
Faction/reputation improvements
Legendary Marks
Armsday
Vault space improvements
The Taken King was a huge update. For a lot of people, it was the "Reaper of Souls" of Destiny 1.
Sorry I'm not trying to make this personal but I look at that and don't see much. There seems to be a fair amount of repetition, like:
Entirely new area, art and all
New world content in that area
New campaign missions
I'm not sure what differentiates a new area vs new world content. And how many of the campaign missions / strike missions were located in the same area but just had different objectives / spawn points? For the new enemy race, how many different cannon-fodder enemies were there? Excluding bosses / mini-bosses, I mean.
For me, for content it looks like 7-8 campaign missions, 4 strikes, some pvp maps (unclear if these are also on the same maps as the strikes?), and a new raid.
For gameplay they added 3 new subclasses and some pvp modes.
For the rest, it seems like the standard stuff you'd expect in a patch -- minor QoL fixes like vault space or leveling adjustment. Except if I recall correctly, there was also a bunch of new items / gear that made anyone without TTK basically uncompetitive in MP, right?
edit: Basically, it might be a decent $15 expansion, but not the second coming. I don't doubt that for a lot of people actually playing, that it was like the "Reaper of Souls", but for people that "checked out" after realizing how little Destiny 1 provided I don't think TTK made it a suddenly much better game. If anything, I think a lot of the Reaper of Souls improvements (like removing Auction house and changing drop rates) helped D3 even if you didn't buy RoS. But with TTK, if you don't buy it, you're kind of s.o.l. because the new gear is gated behind the content, right?
edit 2: Put another way, if you're not the kind of person who likes to replay content, TTK doesn't add that much, right? Maybe 5-10 hours of gameplay. If you're already a fan of the gameplay loop, then sure it can add hundreds or thousands of hours. I only played D3 once at launch but still felt it was a reasonable value. 20-30 hours and a kind of interesting, if over the top, story. I bought Destiny 1 for $15 and feel the 10-15 hours I spent with it was okay, though I was never motivated enough to actually finish due to the poor story and the requirements to grind levels to move to the next story mission. But at $10-15, I'll admit there was enough content there for someone who doesn't like to replay it.
I think the question in my mind was whether Destiny 2 would be more of the same, that is, a game whose value is derived solely from replaying content again and again, or a game that can still provide value without that repetition. I may pick Destiny 2 when it hits $10 (probably not $15 since I never finished D1) or maybe a trilogy remake in 5-10 years at $20.