Well if you arent familiar with Roguelikes, here's a quick intro course on wiki:
http://en.wikipedia.org/wiki/RoguelikeAll the games i've listed work on the same principles. Izuna was a fun little game. In some ways I preferred it over Pokemon Mystery Dungeon because it had a better use of items and the boss fights were a lot of fun. HOWEVER, the game was marketed as game with FUNNY STORY when in fact there was almost no story. Pokemon MD had much more story. Plus Izuna was pretty short and there was some REAL cheesy things you could do in it.
I've only put 2 hours into Mystery Dungeon: Shiren the Wanderer, and it's definitely the hardest of the ones i've listed. I think Pokemon MD had the fewest penalties and its quest structure was amazing. Izuna was just a go through levels, get to boss, beat boss situation. I havent played enough Shiren to know how exactly everything works.
Another good game of this sort is on PS2:
