Like others have said, depends on the genre. Overall you want to just give a person a feeling for what the game is about. Introducing an ability or two. Introducing how battles are played out. 3 or 4 music tracks max. I agree on cliffhangers.

This one has two patches that are available. One that's an English translation patch, and another that upgrades the graphics a bit. I haven't gotten around to trying them out myself but it sounds like the patches are compatible with each other.

Tengai Makyou 2 is allegedly in the works I think by SamIAm? Someone was looking into both Tenshi no Uta's, but haven't seen him online lately. Anearth Fantasy Stories has been fully hacked but is on the back burner. Emerald Dragon is gradually in the works as well.

There's also a PC Engine CD version with even more added stuff that's not in the HuCard version.