Number 1: The Expanded Roster
All of our favorite characters from the first are returning. But now we get even more. Including, but not limited to, Quicksilver, his sister Scarlet Witch, and the Vision. We also will be introduced to the artificial intelligence, Ultron. Played the great, James Spader.
Number 2: Unique Action Sequences
As Joss Whedon himself said, most of the Avengers in the first film are primarily punching oriented. But now we will get heroes that have the potential for interseting action sequences that will have us on the edge of our seats. With Quicksilver's speed, Vision's intangibility, and Scarlet Witch's magic it could give "[X-Men: Days Of Future Past](movie:203942)" a run for it's money.
Number 3: Joss Whedon Is Still Involved
There's talk that Joss will not return to direct either of the "[The Avengers: Infinity War](movie:738027)" films. Even though he is the perfect man for all the Avengers films. Since he is good at directing large casts ("Buffy the Vampire Slayer" & "Firefly") and doing big epic season finales ("Buffy the Vampire Slayer"). Each Avengers movie being a big season final to each phase of the Marvel Cinematic Universe.
But, like I said, he won't return. Making "Avengers 2: Age of Ultron" his series final. So he will most likely step up his game and do his best to say goodbye to the MCU.
Number 4: Hulk vs Hulkbuster
The first film gave us many superhero fights. Both verbal and physical. Hulk vs Hulkbuster promises to be bigger and better than all of them. Nuff said.
Number 5: It'll Lead to Something Bigger
Phase 3 is set to contain a total of 9 films. Making it the biggest Phase in the Marvel Cinematic Universe. Introducing us to numerous new characters. This will marked by "Avengers 2: Age of Ultron".Son it has to be big and bombastic to mark the occasion.
All of which leading to the Avengers biggest challenge the Avengers will face. The most powerful being in the universe, with the most powerful weapon in the universe. Thanos and the Infinity Gauntlet.