You pay for your game and expect it to have a certain amount of content. There are many ways to pad that with fake longevity, most of which amount to adding gameplay flaws to hide the “flaw” of being short. Character advancement is the currently popular version, because grinding takes time and your players are happily watching their numbers increase.
Dungeon Defenders has more real and fake longevity than Orcs Must Die! The longevity extension measures, however, are better in Orcs Must Die! These are both action/tower defense games released around the same time, so they provide a good basis for comparison.
Both games have scaling difficulty levels. Both games unlock abilities over time and let you upgrade your stuff. Both games have a series of maps that take you through a campaign, and now both have DLC maps to extend that.
Dungeon Defenders has more real longevity because it has different classes, it restricts options in a way that promotes replayability, it is a multiplayer game, and it has challenge modes. The different classes play differently, from towers to abilities to attack forms. The towers are spread across classes where the Orcs Must Die! warmage can pick any ten. This means that the map plays somewhat differently as an Apprentice or a Monk, whereas it is unusual to have an Orcs Must Die! map where you might want more than ten options. (This difference is reduced if you have multiple Dungeon Defenders characters and swap between them.) Multiplayer done well means more fun, because everything’s more fun with your friends, and having more moving pieces gives you more permutations. Finally, the challenge modes recycle the maps with different game mechanics. A recent DLC explicitly recycled, adapting the warping core challenge to more maps.
Dungeon Defenders has more fake longevity because it has a level and equipment grind. If you just want to run through each game once, no problem, the Dungeon Defenders difficulty curve has no huge lumps once you have all your abilities at level 20. If you want to beat the challenges or play at the highest difficulty, you need to grind to max level, you need to grind equipment, and you need to grind mana to upgrade your equipment. Orcs Must Die!, contrarily, lets you upgrade each trap exactly once. You cannot upgrade them all before beating the game, but you will have enough skulls to upgrade your favorites by the last map. Each map must be balanced properly because you cannot grind the previous levels to overpower the next one.
This forces discipline on the developer and a necessity for thought on the player, which makes the longevity measures in Orcs Must Die! better. Each level you are rated on a scale of one to five skulls: one for reaching the “par” time and then four skulls for beating the level without letting any enemies enter the rift. This sets up the obvious goal (and achievement): get 5 skulls on every map. This is also how you get more upgrades, so you must improve your play to upgrade (instead of upgrading to improve your play). Both Dungeon Defenders and Orcs Must Die! have scores for their maps, but you know that you are facing a level playing field in Orcs Must Die!, rather than trying to top the player who ground out a half-billion mana to upgrade equipment. Orcs Must Die! also has its scoreboards in-game and listing just your friends, because that’s who you really care about beating.
Extending the life of your game by adding an indefinite grind taxes your servers and creates unhealthy incentives for your players, although it certainly works nicely for getting more play time logged. Extending the life of your game by focusing on the strategy aspects and rewarding better performance is good, healthful design.