I've played all the assassin's creeds save the handheld ones, and I came into this one with a lot of promise that ultimately just turned into frustration over numerous step backs in gameplay.
Pacing wise I think ACIV is vastly better than III, and stability wise it likewise is WAY ahead. Visually, it is also an improvement, and story over all seems a bit better as well. However, all that little matters when the actual game play is sadly a step back.
Combat has become a MESS! Most the combat you'll see is while raiding other ships, but this shows just how incredibly poor the combat engine is for the game. Your own ship mates get in the way. Counters don't respond. You'll be attacked from off screen. In certain situations you can be essentially 1 shot due to the silly mechanic that you can be knocked off the masts very easily yet you are forced to go up there blind. Assassin's creed has never had strong combat mechanics, but this nothing but frustration. And, adding insult to the whole thing is the issue that unless you execute a finishing move, you can't kill people, but a good amount of the time you'll be interrupted from such by your own men resulting in you slashing away on a lowly peon for a good 30 seconds or more and not killing him. And unfortunately they have done nothing to fix the ease of combat when you're not surrounded by your interfering own people.
Next issue is the ship mechanic. A lot has been taken for III, but they switched to this system then every shot type is based on position you're looking at... sorta. I've had many times when I'm facing to the side, yet by bow guns fire. Other times the opposite occurs. I think it is due to the last cannon aimed it the one that fires, but it is just annoying. Most the rest isn't too bad, but the inability to scan around you quickly to see terrain is another weakness, and the whole system feels like a step forward and back, and not really solving any of the problem III had in this regard.
A number of other issues just continue to annoy. They brought back these silly chase the page mission, which were nothing but annoying in the previous game. Everything costs a ton, even fairly basic upgrades, forcing a lot of grinding. The shipping mini-game is needlessly slow making it rather pointless in the long run. The game refuses to do things at various (fast travel, crafting) and doesn't tell you why, it just won't let you. There is some really odd choices at time for check points (particularly evident in forced stealth missions). Also, it quickly gets repetitive with only a little variety in tasks that are unfortunately quite slow due to the setup of the game (you have to jump off the ship, swim ashore, do whatever, swin back out... It just gets tedious very quickly).
Funny enough when you are on your own it shines through again fairly well which is what save it from a lower score for me. Sadly, contrary to popular opinion it seems, I don't think I can put this actually overall above III. While III had a lot of issues, IV has just as many, they are just different, and in my opinion more annoying.