Assassin's Creed 1.5?

In short, no, but let's face it, the game's core is nothing new from the previous, it's everything around it that has changed. Assassin's Creed had an appealing story and the somewhat interesting character Altaïr but it the lack of content around it made it incredibly tedious and bland to play through. Don't get me wrong I plowed through that sucker with a gleeful smile to my lips as I cut fools from left to right out of simple mischief.
The problem was that I had to create my own challenges to survive the boredom of doing yet another flag run or pick pocket missions. Like playing with a hand and a foot or trying to complete a side mission without blinking my eyes. While that in it's own can be fun it's not exactly the experience I payed for. I could just go outside and pretend I have friends in that case.
I'll leave it with that and continue with my impressions of Assassin's Creed 2 and why it's not just a 1.5.
Desmond, meet Ezio

From here on Desmond and his new friends will only make a small number of appearances compared to AC I but they feel more natural and interesting. Instead we're following the story of Ezio Auditore di Firenze, the Italian nobleman of Florence. Unlike Altaïr, we're following him from birth all the way to his thirties and most importantly through his youth where a big portion of his family is betrayed and sentenced to death.
This is where Ezio must assume his father's role as an assassin and uncover the truth behind the templar conspiracy. And it's this that draws me into the story because it feels like he's properly learning how to be a cold, blade wielding assassin and it's more clear to what drives him.
The Renaissance Men

He'll supply Ezio with tools such as the twin blades, hollowed out blade for poison attacks, increased health and even a small wrist mounted gun to really kick the myths behind the inventor into gear. This offers huge motivation to explore the cities and uncover map sections to find the codex pages. That together with the subject 16 truth, a series of video snaps that together offers a startling revelation. These are hidden behind glyphs that are strewn throughout the cities, finding them can feel a bit tedious but the building descriptions are kind enough to alert you to the proximity of one by an icon of an eye in the corner of the picture.
The Game he Plays

There's also more tools against the guards, toss gold on the ground and some of the lower ranked guards runs up and begins to pick them up while the officer is left to be killed. Or throw a smokebomb and you can calmly leave the area while they cough and wave their arms around blindly. It just generally feel like you always have the means to get the upper hand on even the strongest of opponents.

Sometimes Ezio really work against you.
I got a job for you. it's different, I swear
The mission structure is the same like the first game, run to a mission trigger and work your magic after the ingame cutscene. What stands out here is the fact that the work around you do before you go in for the kill of a major character feel more different and weaved deeper into the story. Instead of doing your fifth pick pocketing mission for some shred information, you sneak around the canals of Venice to acquire guard uniforms or you tail officials by using hired courtesans as a cover.There are still proper side missions but they also feel more worked on, like assassination missions that can wary between killing people in unarmed combat or tossing them off rooftops to make it look like an accident.
Money, feathers, Villa!

The Villa will also become your gallery of stuff you collect throughout the game. Paintings are hung on the walls as you buy them, armor pieces that no longer are used are placed on dummies and weapons on racks for display. It genuinely feel like you do progress even while doing tedious feather collection as there's in-game feedback and reward to everything .
(not to mention the kindness from the developers to show exactly how many of what you've collected in each city)
Final Words
Assassin's Creed II is a treat throughout and I had a great fun doing everything, including the feather hunting. The Villa motivated me to scavenge entire Italy for the missing pieces and the clues within the Truth was incredibly interesting to dig through. Even if it felt like Ezio's story fell a bit in the backseat at the end it still felt interesting as it was delivered in such a way to describe the future of the assassin. Being able to play the game with Italian voice was interesting and a thankful way to add more depth to the renaissance era you play in but something I eventually switched back to English for more ease in following dialog.And there's no real replay value apart from continuing the game to find all of the collectibles for achievements, but I still felt a certain level of fulfillment by the end of this game instead of the otherwise emptiness. DLC to continues Ezio's story a bit more would always be welcome but it's not a necessity to increase the value of this game. While there are some hiccups in the movement and some annoying repetition in NPC dialog, I'm satisfied and it was worth every cent in my book.
I would even dare to go as far as to say this is MY game of the year in terms of experience and feel while playing.
Played Assassin's Creed II on the Xbox 360, using the Black Edition extra content. Completed it in around 50 hours total playtime and had collected everything but the feathers before the final Mission.
Played an extra 3-5 hours to collect the full 1000/1000 achievement points.