Having reviewed the game for NWR I agree AND disagree with some of your points. I agree that the game's storyline is a tad more serious than the first one, but I feel it's still ridiculous enough that it never gets pretentious. In my review, I compared it to a Quentin Tarantino revenge film in which the character goes on a very personal and deep journey of revenge, but does it in such a ridiculous manner than any hints of serious storytelling gets blocked by insanity and eye candy. Think of it as being the male, videogame version of Kill Bill.
The game does feel a tad easier, but it should be noted that it has multiple difficulty settings, and if the last boss battle is any indication playing the game at a higher difficulty setting should be an enjoyable challenge. Also the extra polish given to the gameplay also makes it that much easier to learn and get into.
I disagree in that the assassins are not as engaging. They are far more ridiculous this time around (I mean...Kevin McDonald has mecha cheerleaders!). I think what you mean is that the game doesn't give them enough presence to make them memorable. In the first game, whenever you entered a ranking match you read the assassin's name along with a creepy shadow of them, perfectly setting the mood and making the battle feel like an event.
I think that with Desperate Struggle Suda51 really wanted to address some of the complains about the first one. You should note that A LOT of people had issues traveling around Santa Destroy, navigating Travis's bike, working to enter the matches and many other things. So the sequel feels more straight forward, apologetic in many ways.
Suda51 and Grasshopper are eccentric game makers, often making games that don't make sense or aren't accessible from the get go. Killer 7 is proof of that. Desperate Struggle may be the first game that actually tries to be easier, more inviting and more forgiving, whereas the first one YOU had to learn its rules and learn to enjoy it. In many ways, we have gotten used to the crazy game design that seeing a more straight forward product out of them feels even weirder.
But I am rambling now. I think it's one of the best games of the year yet.