It's in Revelations People!
Revelations is the uninteresting rooftop after the steep, beautifully-animated ascents of the majestic cathedrals that were AC 2 and Brotherhood. Now that the free-running analogy is out of the way, take a leap of faith and read on.
The steady improvement of the Assassin's Creed series had to slow down eventually and AC: Revelations is where the buck stops. AC2 was phenomenal, all the potential of the innovative but repetitive first game fully realised. Brotherhood was the unexpectedly enjoyable "AC 2.5" that came out only a year later. Another year later and we had a true franchise on our hands in the form of Revelations, and at last the great leaps in quality came to a halt.
Instead of just refining the already introduced mechanics of previous games, they added a whole lot more, none of them all that fun or creative. These include a bomb-crafting system (it's the Ottoman Empire after all), a truly out-of-place tower defense mini-game, and the hook-blade, an upgrade to the traditional hidden blade that allows zip line travel and... not much else. Instead of feeling like a fresh way of playing a badass-stabby-old-Italian-bloke, the new features just seem like something to fill out a list on the back of the box.
The titular feature of Brotherhood: recruiting and upgrading your own team of Assassin pupils, in Revelations is something that has lost its flair. In the previous game there was a single Assassin Den; a cool hideout where you could upgrade armour, weapons and show off your collectibles. In Revelations the dens are everywhere and every one of them has its own loading screen. The already mentioned tower defense is a repeatable mission to stop these dens being lost to the enemy, and when you've done it once, you've done it a hundred times.
One of my favourite things about AC2 was the way they handled the parallel present day storyline of Desmond Miles. This continued in Brotherhood with the team moving to present day Monteriggioni, a nice touch. In Revelations (for plot reasons I won't go into) this side of the AC story is completely absent. They also ditched all the riddles/puzzle solving from the previous two games, which I loved. We get some brief voice overs from present day, some really odd first-person platforming segments, and we finally see Subject 16 in the flesh, but none of it comes close to the crazy sci-fi aspects of previous entries.
The 'revelations' themselves take the form of short sequences where you play as Altair (the hero from the first game) at various points in his life. These sections are supposed to wrap up every single loose thread about Altair and Ezio in preparation for the clean slate of Assassin's Creed 3. They're a welcome distraction but once again they feel unnecessary and certainly aren't as revelatory as one might hope.
Revelations undoubtedly looks good on PC, and of course the art team does a very good job capturing the atmosphere of Ottoman-era Istanbul. But it sure as hell isn't Venice or Florence or Rome. The same sense of history just isn't there.
The game is good, not great. At around 25 hours long you'll get your moneys worth, but its just not as ground-breaking or mind-blowing as its predecessors. Bottom-line: Unless you can't help yourself and have to play every single entry in this series (like me), I'd wait for a Steam Sale.