2. Evolve is the Version 2.0 of a Left 4 Dead Finale
Some of the most intense parts of Left 4 Dead are the Tank fights during the Finale sequences of each campaign. These sections pits players against two Tank Infected before the campaign end is reached. Finale's required the most cooperation than at any other given time, mainly because the Tank has the power to take out a Survivor team single-handedly if they don't work together properly. Granted, the three other players on the Infected team could still assist the Tank in fighting the Survivors, they definitely weren't the primary targets of the Survivors. It was easy to see who's the more threatening enemy when the Tank can eat hundreds of bullets, knock around instant-incapacitating cars, throw huge chunks of rock with great accuracy, and punch Survivors off ledges. However, in higher skill levels of the game, it was up to the player controlling the Tank to stay alive as long as possible and avoid being lit on fire with a molotov cocktail. Often times, Survivors will corner the Tank on the map and light it on fire, forcing it to go on the offensive.
Evolve is essentially these fights with Tanks on a much broader scale. The Hunters are out to locate and destroy the Monster before he can grow and become too powerful to handle. Alternatively, the Monster needs to stay alive long enough to grow more powerful and destroy the main generator in the Hunter's base (wins the game for the Monster). The added dynamic of Hunters trying to pursue a hulking beast while protecting something besides themselves will add tons of new elements to the cooperation found within this game. Should the Hunters split up to pursue the Monster and protect their generator, or should they stick close to their generator as one unit and let the Monster grow more powerful? The choices, the choices!
3. There's More Space to Roam Around
Left 4 Dead's maps were basically courses that the Survivors needed to move through to reach the next stage. Depending on the chosen campaign, players would eventually see the best routes to take and know where to look out for the most devastating Special Infected attacks. The Finale were also mainly in an enclosed area where backtracking was either immensely difficult to pull off or simply impossible. This was because the Survivors were to make a last stand or sorts while waiting for some type of rescue transportation to arrive. There's not much room to avoid the large number of zombies encroaching on their position. At the same time, the Tank player had limited options for running and hiding if needed. And this is where Evolve seemingly trumps Left 4 Dead.
The maps in Evolve are absolutely immense. To get a sense of what I'm talking about, the Hunters all have the ability to track the Monster's footprints to find its location. Additionally, the Trapper is there to prevent the Monster from fleeing into the wilderness rather than fighting the Hunters head-on. Along with multiple ways for both the Monster and Hunters to move around in their environments, the maps needed to be larger to accommodate these mechanics. Examples being the Monsters being able to climb almost any surface or Hunters being able to use their jetpacks to reach new heights. Without knowing where the Tank will go exactly all the time, the Evolve maps do have many new opportunities for the Monster to hunt as well as be hunted at the same time.
4. Alien Animals/Plants Are More Useful Than the Horde
In Left 4 Dead, the normal Infected are more there to overwhelm and slow down the Survivors for the Special Infected, rather than actually be used to take down the Survivors directly. They are, in a nutshell, small roadblocks that sprint right at you the first chance they can. They aren't intelligent at all and can easily be taken down with a melee weapon or a few bullets to the head. The Special Infected team mainly uses them in the initial strike from a Boomer, blinding the Survivor team and spawning a large swarm to do as mentioned. Killing them doesn't add any more benefit than to clears paths for dealing with the Special Infected hassle-free.
While not as abundant as the Infected, the wildlife in Evolve, however, has multiple functions within the game. First, they are is there to help the main Monster grow in power, allowing it to be able to assault the Hunters directly with much greater success. I believe I read somewhere that the Hunters also use this wildlife to develop their own in-game perk as well, so there's an incentive for them to take them out. Additionally, some of the animals and even plant life are aggressive and will attack Hunters if disturbed. Strategically, the Monster can lure players into dealing with the wildlife to possibly provide an opening for an attack.
What's more useful to you as a player: the same generic and predicable zombies that attack with no common sense, or a wide variety of creatures and killer foliage that are intelligent enough to defend themselves?