AC1 wasn't a great game so obviously Syndicate is far better than that. Don't forget AC is a yearly franchise - how many games have we had since AC2? The bottom line is that Syndicate still has extremely basic combat, terrible AI and less than fluid traversal. After 6 or 7 games the same weaknesses haven't really been addressed. The mission design in Syndicate is nothing special at all, and the assassinations are still as disappointing as they've ever been. The train heists are a terrible missed opportunity - they could have been awesome but in Syndicate it just boils down to finding 4 chests. They've added some optional objectives to the major plot assassinations but they didn't really add depth. Yes the game has become more refined, but at its core I don't see any major strides.
Combat is still press X most of the time, and counter the other guy who when he occasionally attacks you. It needed a complete overhaul several games ago. Traversal should be extremely satisfying in a game like this but it's not. You still can't manually jump - just try getting all the Helix glitches to see how clunky and rigid it is. People like the grappling hook because it completely bypasses half the movement required.
Don't get me wrong, I like the game a lot. I just think that a game with basically infinite resources and 1000 people working on it should be advancing more rapidly.
Except you're over simplifying things and hand waving away changes. Anyone who has played Syndicate would tell you the traversal is greatly improved over 1, and hell over Unity even. Outside of a small handful of hitches here and there I never ran into a single problem with the traversal, and worked about 1000x better than it did in 1.
Most of the things you're complaining about are opinions and not facts. I loved the new mission types in Syndicate, they built on things like clearing out a nest and added interesting changes, like freeing children. Hell the murder mysteries carried over from Unity saw huge improvements and made that NEW mechanic even better.
I don't know what people expect either, the game is Assassin's Creed. Your missions are going to be stealthing around and killing people. They've added ways to poison enemies, ways to attack from afar, and other introductions that change things up. The base idea of the missions are always going to be sculking around and killing people in hiding though. I mean it's like complaining that Mario just keeps platform jumping . It's a silly complaint to me.
Except in AC1 combat wasn't press X most the time and counter the other guy. It was a lot more complicated, and way less refined. Plus if all you're doing is spamming X and counter you haven't bothered to try and learn the combat system. There's more depth there, but if you decide to ignore it, that's on you.
Helix glitches are physical puzzles to be solved. It's about finding the right angle to approach them from.
I recently went back and played through a bunch of AC1, and then started on Syndicate. There have been some huge refinements to the series, anyone saying otherwise isn't paying attention IMHO.
The series tried lots of different things in a lot of the releases, some worked and some didn't. I think that just because it comes out every year people lose perspective to how much the series has changed over time.
I mean shit you had full on sailing simulation, with new missions based around those for 3 entries in the series.
http://blog.ubi.com/what-each-assassins-creed-brought-to-the-series/
*edit* plus hell in Unity you had multiple ways of accomplishing the major assassinations, and they brought that back and refined it for Syndicate. Every major assassination had different ways it could be accomplished. One of the best assassinations in the franchise was the one from Unity where you replace the priest in the confessional and kill the guy when he comes to confess.