It's just a really well done take on those games. It has a wide variety of items and classes to choose which varies the play a good bit, online and local co-op, and a nice difficulty level. I haven't played Nuclear Throne yet and haven't devoted nearly enough time to Teleglitch to give it a fair assessment, but so far I'd put Risk of Rain below Isaac and above Rogue Legacy because it doesn't have the progression tree (it has item unlocks much like Isaac, where you complete tasks in game to get items/characters in future playthroughs). The feeling of being overrun by enemies that Teleglitch provides is there, but you don't need to worry about ammo/resource management. The sci-fi setting is nice, the enemies look pretty good despite the very simplistic graphics.
Spelunky is still the best game that's been made.
