According to the devlelopers:

-Seamless building interiors, roughly 30% of the buildings in game can be entered as well.

-Increased number of characters on screen, up to 1000. AI characters in crowds now have more variety and will act differently instead of all acting as one big group.

-Drop in, drop out 4 player co-op added to the main campaign

-Parkour has been divided into parkour up and parkour down. In other words, you can now easily drop downwards on buildings, which you couldn't before.

-There is now a stealth button, which allows you to crouch and enter cover, and reduces noise and visibility, for increased stealth ability.

-They have added a skill tree, though it's mostly cosmetic improvements for the parkour system.

-Combat difficulty has been tuned to make open combat more challenging, encouraging players to return to the series' stealth and assassinations roots.

-Players now have a hidden crossbow in addition to the hidden blade

-One of the biggest AC maps, Paris is bigger than all of the playable landmass in AC 4 combined by a decent margin.

AC Unity was basically about them going back to the series' roots of stealth, assassinations, and parkour, and refining those basic mechanics since they've grown a bit stale. It was also about bringing the series properly into the next-gen, as such they spent a full year working on the graphics engine. It might not be as big a leap as 4 gameplay wise, but 4 was drastically straying from the series' roots anyway, it was a pirate simulator, not a game about Assassins. It was fun, but it didn't really feel like an AC game.

Unity should be pretty close to being done, more than likely they're done making content and are just bug searching and optimizing now. Not sure about Comet since they still haven't officially announced it. I get the feeling they delayed it to make next-gen ports because they're anticipating low sales as a last-gen only game. They'll probably release it in Q1 next year across all platforms, probably as a cheaper downloadable game like AC Liberation HD was. 

I don't see how this is bad. Unity was in development for 4 years, comet probably for some years as well.
I would be really worried if there wasn't another AC in development.

People complain about yearly sequels but completely ignore that all of these games had the same development times than other AAA games. That's the reason why they can maintain the high quality with each game.

I would be really happy if other developers had that kind of money and manpower to release my favorite franchises on a yearly basis.

Same here. I so wish Nintendo would release my fav franchise, Mario, on a yearly bases......

Oh wait, they do ))

Haven't seen a Mario 64 sequel yet and it's been some time.