I like 1840 a great deal. I think DS revved up again and redeemed itself with 1840, after spinning its wheels painfully with the preceding few storylines.
PROS:
Gabriel, and all the Gabriel-Gerard-Samantha intrigue, especially at the beginning
Ang fresh from 1795, instead of time-jumping Ang
Barnabas as his 1840 self
1840 Ben (we hardly knew ye)
Julia knowing more than Ang for once
Lamar's final moments
Lamar's ironic profession
Daniel
John Karlen mastering his mainstream accent, Desmond the hero in general
Anything and everything about The Head.... the nauseous horror of it, Desmond's cluelessness about whipping off the cover and showing it to everyone.... Flora's weak flustered sputtering about how she'll never believe that this disembodied head is just some innocent little knick-knack....!!!! Julia as Frankenstein, only the "experiment" is genuinely scary this time.... GREAT new piece of music that accompanies the Head, especially that dark, surreal "swirling" effect at the end of that music....
The complicated, surprising resolution to the Barnabas/Ang story, with her reformation etc.. See elsewhere for any remarks of mine on why it worked.
Barnabas showing up in court after Trask had bricked him up.
The rationale behind the Staircase. There is no time, only physical space.... that's fascinating. I like trying to wrap my head around that. I doubt that could be an original idea on their part, but it's the only place I've ever heard it.
CONS:
Continuity: As always, Barney no go back in box at end. That's just incredibly sloppy and crazy of them. Ang killed in 1840, though that was dramatically great.
Quentin being framed all over again. Witchcraft trial again.
Gerard while possessed. I preferred Gerard the human scoundrel, and the real source of genuine horror, the Head, was out of the picture.
Backdating Ang to the 17th century. After a century of witchery she decided to work as a servant in Martinique?
I'm sure I have more complaints, but can't think of them. I like 1840. The repetition of 1897 elements is annoying, but everything was stolen or reused ad nauseum on DS, I'm finding out, so I'd better find a way to deal with it.
The confusing origin of Roxanne as a vampire... we don't usually see the present-day consequences of Barnabas's actions in the past, before Barnabas has gone back and made those changes, in his personal time-line. I like it, it's just inconsistent.
NOT A PROBLEM: Lack of Tad and Carrie toward end. Recently it hit me that all the stuff that made Gerard/Judah's ghost so bitter and caused all those ghosts to haunt Collinwood in 1970-- all that happened in 1841 or later, after Judah was master of Collinwood. All that was prevented from happening... of course we don't see those events. If we did, the problems wouldn't have been averted, and they were.
Roxanne being killed in 1840. I'm not quite sure how this could be a problem. Time-travel mission accomplished, that's what it indicates to me. The problem with her as a vampire in 1970 is over.