Seeing as someone asked: yes, Keep on the Shadowfell does leave a few pieces unresolved.
The Rift: The module has nothing to say about the eventual fate of the Shadow Rift. The simplest answer is to declare that Kalarel's death ends the ritual and closes the Rift, although that explanation leaves one wondering why a garrison was required in Sir Keegan's day and how things are going to go without one in the future. A better answer, although sadly not within the scope of the module as-written, is to let the players close the Rift forever using some item they've found on their travels (Aecris, the ancient mirror, or one of the various Bahamut relics) and/or have the Keep collapse and bury the Rift for all time.
The Other Monsters: Presuming the players didn't clear every room of the Keep, there are still monsters hiding in the corners somewhere. It's incredibly anti-climactic to follow up Kalarel's defeat with a mop-up session where the players go hunting isolated pockets of low-level trash. Again, there's no help from the module - in fact, it seems to specifically envisage this mop-up in some encounter descriptions - but if you don't go with the "collapsing Keep" scenario described above, another option is to have the Winterhaven militia finally do their job and roust out the remaining baddies. Remember to add any loot your players would have missed onto their end-of-adventure reward or you'll be encouraging exactly the sort of downbeat denouement we're trying to avoid.
Winterhaven: As written, there's no good reason for players to hang around Winterhaven after they've picked up their rewards. The module suggests the village might form a useful base of operations, but contrast that with the fact that on a good day the shops stock some first-level magic items and there's no other significant adventuring locations within two days' walk. Obviously Eleven Foot Pole is going to be moving on to Thunderspire Labyrinth but for players who've really enjoyed Winterhaven and want to continue hanging out with their NPC friends there, there's really not a lot of support.