This enigma riddles are probably my favorite part so far. Really like how they get you to get to know the city. No tail missions are great, but still some insta-fail so far, the early PR made it sound like they were removing the insta-fail and it'd be more like MGS where any situation can morph into a new situation if you're detected instead of just restarting from checkpoint.
Modern day stuff is a shame, probably the second worst in the series, only beaten by the FPS platforming of Rev. Time rifts seem to be the "modern day" replacement, but they lack the story context.
One thing I don't get is how since AC:B they haven't been able to capture the economy/upgrade your base system again. ACIV was visually cool base-building but it didn't really do much except use all your money, AC2/B did it great with walking through town upgrading shops, buying paintings to display, increasing everything in your Villa to bring in more money for yourself. I'd really like a nice upgrade system like this again some day in an AC.
Would like to see them take this engine, which is gorgeous, and scope and add back the ocean and sailing for an even bigger scope. Have a game like Unity, but with a smaller Paris at the expense of a second major city that you can sail to with islands and things in-between.
Also, lack of deathmatch, versus MP modes is a bit of a shame with such a huge Paris and so much space to make fun cat & mouse chases. They should do a multiplayer focused DLC expansion that adds versus game styles to play in the city.