Late to the party, but it's weird to hear the Sonic Adventure soundtrack called pop punk - I always thought of it more as hard rock/funk. There's a pretty strong funk influence in the character select and Hot Shelter themes, for example.
Y'all, this soundtrack is HOT. Jun Senoue's tone, rhythm playing, and the active basslines are my favorite things about this soundtrack. His rhythm tone isn't too thick or too thin and has a heat to it, and his lead tones are incredibly smooth. He also adds a lot of rhythmic accents using pinch harmonics and not-quite-pinch-harmonics, and pushes against the tempo a bit to add interest. For an example, listen to the rhythm line in the character select jingle during the solos. If the rhythm guitar and drum parts were people, they'd both be dancing and the guitar would be grinding on the drums. That degree of rhythmic variation really isn't common in metal or hard rock - bands typically stick to varying between muted and unmuted notes, and every now and then throw in a full pinch harmonic with some vibrato for attitude - they can't take too much attention away from the vocals.
I also really like the active basslines. You can actually hear what the bass is doing, and it doesn't just shadow the rhythm guitar - it's a unique part that still complements what's happening in the rest of the song, but without taking too much attention away from it. Best example of this would be the Sky Deck theme. Note the hints of influence from surf rock and boogie.
This soundtrack feels like summertime. None of the other soundtracks have captured that same feeling - even SA2 doesn't have the same heat and attitude.