The criteria for a good sequel is almost contradictory; it's a dangerous mix of old and new, with just enough old to serve as a sturdy foundation and just enough new to prevent boredom and apathy. But when not perfectly balanced, a sequel can prove disastrous -- and said franchise may later find itself effectively neutered, with no chance of future offspring. Rely too much on the previous game and you'll have players wondering if that Roman numeral at the end of your title is just a sleep-deprived hallucination brought on by attending too many midnight launches. Abandon the predecessor entirely and you'll alienate those who spent months of their lives posting in message-board threads and feverishly stroking their pre-order receipts.
If you're thinking it's a wonder that good sequels even exist, then you're right -- and the following games will prove why you're such an astute observer of the gaming industry and no stranger to disappointment.