I like the party play mode. If you have the game just sitting idly at the title screen, it will be playing a random song, and a person can jump in at any time and play whatever instrument or difficulty they wish. I also like the challenge mode that gives you tasks like 'get a score over XXX' or 'hit so many notes in a row'. It helps add replayability to songs you wouldn't play more than once.
The vocal system sucks because you have to be pitch perfect to get credit for any section of a vocal phrase, and the feedback is mediocre at best. There is a glowing dot and you have to match it exactly to the lines. The arrow in RB is better because you can see the tail end and adjust on the fly if you are slightly off, whereas it is a guessing game in GH5. In GH5, a bar fills up, and when it hits green, your multiplier increases wehereas there was no visual feedback whatsoever in GHWT.