Joss Whedon did a fantastic job with Avengers
, IMO. All the characters had moments to truly shine, the action was amazing and it left you feeling great. Ditto Abrams with Star Trek
. Compare with the pre-2009 Trek movies, where they rarely gave thought to anyone outside of Kirk/Spock and Picard/Data. Now imagine a Trek mashup movie where ALL the crews (surviving members in TOS' case) come together. All the modest budget goes on Shatner, Nimoy and Stewart's pay cheques so no big expensive sequences. A director not as good as Abrams or Whedon. A Berman-era Trek mindset behind the whole thing.
It might have been okay. But after seeing Picard and Kirk's meeting in Generations
(where they rode horses together and then killed Kirk) I see a far greater chance of failure.