Replay value means one of four things for me: Good multiplayer, Sandbox, Procedural Generation, or Mods.
It only needs one for replay value*.
Good Multiplayer, I keep going back and playing it, there are no end conditions to killing other people, you just keep playing new matches.
By sandbox, I don't mean open world, I mean Garrys Mod, Dwarf Fortress, Minecraft, and other creation games. An open world allows me to play longer, but I still get sick of the gameplay faster than I am including in replayable.
Procedural Generation. The weakest of my replayables, because I was going to use Dwarf Fortress and Minecraft as my examples, however they are also fully open into what you will do, so I put them under sandbox, despite DF being quite different from Garrys mod style sandbox. Nethack also goes here, which is cool.
Mods. I've played Oblivion a lot, always deciding to play it again because of some cool mod that looks awesome. Valve is my favorite company because of their modding community (many source mods are better than full priced games). I keep coming back to Mount and Blade, especially after seeing something in their forums I had to download and try. All of these have modding communities that change the game drastically.
Of note, multiple endings aren't enough for me to replay a mostly linear game, and I find achievements pointless.
*the game has to be good in the first place of course