Part I: A series you wished had gone on longer or one you wished would just END ALREADY (or both!): I would read Diana Wynne Jones' Howl books until the world ended. I don't know if that's healthy or whatever. But it's true. It's unlikely we will get any more.
Series that need to END are Paolini's Inheritance "trilogy" and Robert Jordan's Wheel of Time series (which also started as a trilogy). Both series have major weaknesses, but I also love them to bits and would like to see some CONCLUSION.
Part 2: A series or book that you wish more people were talking about and reading: Hmmm....hard game. Well it's actually quite hard to find some good old Henry James lovers....I keep gushing about the poor man and everyone thinks I'm insane. I would love to discuss his books and themes in-depth. Also I was very sad for a while because I didn't know anyone personally who had read Howl's Moving Castle, despite its utter MAGIC, but that's solved now, yay! The Perilous Gard and Sherwood's Ring by Elizabeth Marie Pope are, as far as I can tell, extremely under-read and underrated.