It's got that issue that the difficulty curve is largely player-defined.
You can complete nearly the whole game with basic attacks, counters and combat executions. It'd be dull as anything (and I think a lot of the people finding the game shallow are playing this way) but it would work. You could also spaz your way through all the real missions and ignore the side quests/vendettas/nemesis system and 'complete' the game that way. Stealth, unless specific for a mission, is not really punished for failure, you just get a big fight. Which you can win with basic attacks, counters and executions.
If, on the other hand, you decide for yourself to get into the character, try different approaches, use all your different attacks/tools, try not to be spotted, pick an enemy captain, boost him through the ranks, then kill him, get the side quests, work on the highest crit combo you can etc etc then there's a lot more game to be had.
Anyone who plays games in the most efficient manner will probably not have that good a time with it. I realise it's a valid way to play, and for some people any alternative prompts the question "why should I make it harder than it has to be?". I'd answer "because it's more fun" but that's entirely subjective.
hit the nail on the head there, I'm also trying to complete all the side missions and enjoying getting the most out of the game, I never understand those that just rush through a games main missions completing it in a couple of hours and then complain it was boring.