I guess I'm in the minority, as it's a FULL DAY park for me!
When I get there at rope drop, I am NEVER able to finish the park in anything less than a full day! And the only times I've been able to do every open attraction on the same day is when the park closes at 10 PM with a second Fantasmic! show at 10 or 10:30 PM.
And even on most 10 PM closes, there are usually one or two attractions I end up having to skip. Being a "show park", where many of the attractions are shows that take place at scheduled times, it can be a challenge to line up all the show times to make sure you get them all in, and this requires a lot of backtracking and walking across the park. Also, since most of the attractions have long run times, if you find yourself with 20-30 minutes to kill before a show you want to see starts, there is often no attraction that can be done in this time without risking missing the beginning of the show.
On an 8 PM or earlier close, it is impossible for me to do every attraction in a full day, so I don't even try! Finishing every attraction on an 8 PM close would mean having to rush through Walt Disney: One Man's Dream, which I am not prepared to do!
Whatever I have to skip due to time constraints, I always make sure I do on my next visit, so I still get to experience everything fairly regularly that way. Although once a year for some (such as American Idol) is enough (as a seasonal resident, I do multiple full day visits to all 4 parks every year)
Now, when the Studios opened in 1989, I was there the opening season, and back then I WOULD consider it a half-day park. It took about that long to see and do everything thoroughly! But IMO it hasn't been a "half-day" park for a long, long time.
Now if someone does not care for what the park offers enough to stay for a full day, that is a different matter! But I don't think anyone could realistically say that you can do EVERYTHING the park currently offers in a half-day or really anything less than a full day!
Here is a list of what I normally do on each full day visit which EASILY fills up an early-close day, in no particular order (although the first two are usually the same)
Rock N Roller Coaster - multiple rides at rope drop; possible last ride near closing with Fastpass
The Great Movie Ride (and then finding the cement character footprints of Mickey Mouse and pals outside the theatre)
Playhouse Disney: Live On Stage
The Magic Of Disney Animation
a) looking at the character development art in the outdoor waiting area
b) Mushu show, which I find hysterically funny
c) Looking at concept art displays for upcoming animated features
d) Looking at the fingerprint cement blocks in the outdoor courtyard of some of Walt's most legendary animators
e) coloring a character in the postshow area
f) Meeting the current character/s from the current film, (although I don't do this on every visit)
g) Trying to improve my poor drawing skills in the Animation Academy
h) Looking at the art on display in the attraction postshow gallery (currently from Snow White)
So, just this one attraction takes up a bit of time, but then I'm a hardcore Disney animation buff!
Walt Disney: One Man's Dream
a) at least an hour reading every panel, and looking at every single artifact and picture on display
b) Watching the movie
I'm a hard core Walt Disney/Disneyana buff, so this also takes up a large part of my day, but it never gets old for me!
Voyage Of the Little Mermaid
Beauty and the Beast: Live On Stage
Tower Of Terror (with Fastpass)
Toy Story Midway Mania (with Fastpass)
Star Tours (including watching all or at least a good part of the charming banter between R2-D2 and C-3P0, which I enjoy as much as the ride!)
Muppet Vision (including watching the preshow in it's entirety, as I enjoy it as much as the main show)
Honey I Shrunk the Kids Movie Set Adventure - I think it's really charming and love the background music loop
Sounds Dangerous (on days when it's open)
a) The main show
b) the postshow exhibits, especially the headphone presentation with R. J. Mc Bean and Mickey Mouse, which I enjoy as much as (if not more than) the main show!
Fantasmic! - I LOVE this show and it's the perfect nightcap finale, so I don't visit this park on non-Fantasmic! days unless there is something special going on that I don't want to miss (such as the Imagineer presentation for the parks 20th aniversary, which was on a non-Fantasmic! day)
So, there's 14 of the 19 attractions, that a full day would feel incomplete to me without.
The remaining 5 are the candidates for skippage due to time constraints, although if I miss any of the first 4, which I enjoy enough to do regularly, I make sure I do them on my next visit. So what it amounts to is most of the following get done on every other full day visit:
Narnia
Lights Motors Action
Indiana Jones Epic Stunt Spectacular
Backlot Tram Tour/AFI postride exhibit (I don't care for the preshow, so if I miss the preshow it won't bother me)
The last attraction, American Idol, I'm not a big fan of, so it's not a part of my regular "rotation"