The 40.X an forward versions of Dwarf Fortress do an extremely poor job of presenting the challenges that were reliably present in 34.11. Specifically:
- Goblin ambushes are gone. (Not sure what happened to them.)
- Sieges are still frequently a casualty of "world activation" where other sites might draw off sieges from your fort, or they might not arrive successfully. (Some steps have been taken to mitigate this, but even if you adjust all the siege triggers, the game rarely achieves the frequency and reliability of sieges that it once had.)
- The new stress-based dwarven psychology means, in practice, that dwarves are either destined to go insane and cannot be kept happy, or will not break as long as you keep up a bare minimum of happy thoughts. An individual dwarf's propensity to stress is pretty much the only stat that matters.
In short, Dwarf Fortress has had a lot of changes that are very interesting and sound neat, but the resulting gameplay has suffered, because the new systems do not actually produce as good results as the older ones. DF is not as good a "game" as it used to be from that standpoint, even if it does many new and interesting things as a simulation.
The main hope is that Toady will eventually circle back to gameplay, balancing, and tuning the game with an eye to bringing it back more in line with the gameplay experience that was the basis for its appeal, but that is probably years away. That makes sense, because with each major release cycle, he's adding entirely new systems that would each probably necessitate another round of game balancing.
As others have suggested, the best approach is to self-impose challenges to increase difficulty, with whatever building and military constraints make it feel hard again, or else explore mods once they catch up with the current version. The siege unreliability will continue to be a disappointment, sadly. It's worth noting that the newer versions have fixed a lot of annoying bugs, so I'm not sure I'd recommend going back versions either.