Its not for everyone but those who like it do so because of the detailed story line, how the world and npcs seem alive and your ability to choose what type of person you'll be.
And the game has enough variety in the paths during combat to make it interesting. I don't get why some of you guys think every single game should mirror GTA or Crysis. Many of the best games of all time are quite linear. It helps the dev to polish the gameplay and AI. Not everyone wants to make a mediocre sandbox or has $100 mil to drop on a game. The hubs are also 'open'. Bioware has always been one to recycle stuff in their games. ME1 had only 1 style of ship and 2 styles of planetary base. In ME2 they did a wonderful job of making each side mission and each loyalty mission feel and look unique.