The plot was great for the first half of the show. I like how they branched off and had others sing like the Acafellas. It was a great way to add diversity to the mix instead of focusing too much on the kids. Then the second half of the show focused more on the kids and emphasized less on the other characters and their personal lives/problems which can make it annoying when it's just the kids over and over again. Still, great season.
The quality of the show can look horrible when the kids would sing on stage. There are times when all those stage lights (which I assume is causing the bad quality) are filmed with the kids singing, the lights cause this laminated look to the picture that makes it look very sub-par quality. Outside of the stage, everything else looks great.
The extras aren't great except the Jukebox which cuts into all the songs right away and play them for you. That's the only feature I like along with the karaoke and the rest are just lame stuff to fill up the disc, I guess.
Overall, great season so hopefully they'll have more characters branching out and sing more songs to add more development than just those set of kids always having problems because there's only so much you can write before the ideas run out.