im sure 30 minutes isnt anything unusual for MGS, some of the cut scenes on MGS2 (never played 3) were very long. as for them being interactive, i think that basically means the way you could move the camera around slightly with the analog.
The way they dont cut to FMV always makes it a little more immersive to me, you know at any second that control could return to the pad, with FMV you pretty much know are watching a pre recorded part until it stops and returns to game graphics.
With MGS3, you can zoom the camera in and move it around like you could in MGS2. There are also prompts (with a flashing of the R1 button at the top of the screen or a few hidden prompts) to press R1 and see something through snake's eyes. Through this prompt, you can get the view of Snake when he looks through his scope, see some hidden easter eggs, and other stuff in the actual cut-scenes.