People usually are excited because it gives us a view of what is to come. If a Final Fantasy is announced with CGi imaging, you can see the atmosphere of the game and you know what you can expect of gameplay (the basics).
With The Last Guardian, we know it is going to be a hugely emotional experience and the trailer suggests just that, meaning everyone is excited, knowing how the previous games turned out.
Halo.... really, does one need to explain this one? It's a huge franchise, which has kept millions of gamers locked to their xbox, and you wonder why they are excited when more Halo is announced? Funny man :)







