Here is a list of things you've getting for $40 with the taken king:
-A bunch of story missions along with cut scenes this time which tell the story of Oryx, start to finish. His story will end in this expansion, so hopefully there'll be better storytelling. We have to find out.
-Quests which carry on the story after you've completed all the story missions.
-4 new strikes and 3 year one "remastered" strikes with new enemies, etc.
-6 man King's fall raid which bungie claims to be the biggest yet. The basically is the end to Oryx's story since he is the final boss. This is the biggest attraction for the taken king.
-8 new crucible maps. 3 new modes so far but looks like there are more they haven't been announced that people found by datamining.
-40+ new exotics (Armour+weapons) to go for.
-Generally more weapons and gear than at vanilla destiny launch.
-One new location - Dreadnaught.
-Court of Oryx game type, essentially a much more improved and challenging form of public events that you have to initiate.
-3 new subclasses, one for each class.
That's what I've seen so far. There may be some I missed and perhaps more to be/not announced. So yeah it's up to you to decide if all that is worth $40. But i just wanted to clarify that you're not paying $40 for a few story missions. That is simply not true.
For me personally the raid, subclasses and exotics alone are worth the price, but that's me. I think everyone has their own preferences. That being said I am REALLY looking forward to the raid. Should be great to get in there with 6 people after so long since Crota's End.
I would say that amount of content is worth a $40 price tag. My issue though is I have no faith in them doing the story well enough for me to want to invest.