Why are we arguing so seriously about something which can't be taken seriously? It's enough to make someone's head hurt. I'm far more interested in seeing the reboot series of films due to come out. I just hope they actually get it right this time.

A point I'm going to make after watching the video in the original post: It isn't hard to take costumed super heroes seriously in films for prime examples of this take a look towards Christopher Nolan's Batman movies, In fact I'll point out a costumed villain in his films even above the brilliant performances by Bale as Batman himself. I am ofcourse giving a massive nod to Heath Ledger's Joker who was all around one of the best performances and most believable performances I've seen in recent years. Something which could explain the reasons why the Spiderman films were "so silly" from an adults perspective could lay in the age ratings given. I know America doesn't really dwell on age restrictions however over here in the UK it's a big deal so lets use british age restriction certificates to prove my point. Comparing the X-men and Spiderman trilogies here, looking over my DVD collection, I noticed Spiderman 2 (and if I recall correctly, my DVDs are on loan to my nephew the others too) are rated as PG or Parental Guidance compared to the X-men films all being rated as 12. Look at the criteria for these age ratings on the page, there's something I would like to quote on the PG page:

Unaccompanied children of any age may watch. A ‘PG’ film should not disturb a child aged around eight or older. However, parents are advised to consider whether the content may upset younger or more sensitive children.
Basically Spiderman was a kid's film from start to finish of the trilogy. Not an adult movie like Batman or even X-Men which were rated as 12s.