I saw a hologram of will.i.am at DLD in Munich alongside Cheryl Cole roughly 3 years ago. I don't see what the fanfare is about? It seems like we have just discovered it, but its by no means nothing new.
a. As the person above me mentioned, Tupac is dead, but not only that, his death is considered (by some) to be fake, so his "appearance" scratches a very strange itch for some. Had his killer been in the audience, he might've started worrying about having to give a refund.
b. There was little to no fanfare surrounding the event, so when he "took the stage" it was shocking. Imagine how different this would've played out if, for months, we'd been hearing about the hologram performance of Tupac at Coachella? All of the press would've been "Not as lifelike as expected." "Overhyped." Etc.
c. "Cool Factor" -- Coachella is a fairly large event with tons of influencers/connectors & "cool kids", who all ran home and tweeted/blogged about it after it happened...as it happened.
d. 4 Technical Feats for the price of one -- Will.I.Am just taped a performance. The Tupac performance required.
-- a company to digitally recreate him -- an body actor to perform -- a company to then map his face onto the actor -- another voice actor to do the greeting.
This may be the first time that all of these factors lined up at once. You would've heard similar things if the Cirque Du Soleil Michael Jackson show had surprised everyone on the last song with a similar move.
Seeing the impossible is nothing new, making it stand next to a real-life something - thats new.
Edit: I also remember the pinball game "Revenge from Mars" from 1999. http://www.youtube.com/watch?v=hA7Cgzk-y1U