The city and exploration is great singleplayer, but that's probably 1/3 of the experience. The 2nd third is doing the main missions on Hard with co-op (via matchmaking is fine), which is fucking phenomenal, and the last piece of the puzzle is Dark Zone, which is a refreshing, innovative and solo-able brand of player-vs-player multiplayer.
So arguably you're missing out on 2/3 of the experience by not playing it online.
However, the city is BIG, and it has great loot game and really good mechanics, so there's still a lot to do by yourself, though it might be a bit less compelling.
- the shooting is only rough if Destiny is your standard for shooting. The Division has fucking great shooting, but obviously it's not going to beat Destiny which is best in class
- the combat is boring until you play missions on Hard - with 3 others so there are more mobs. Play that way and the battles/teamplay is phenomenal
- did you do many of the main missions? In the main mission's pinkies are more level and environment variation than most games have in their entire campaign. The city itself can get prettu one-note, I agree, but the dynamic weather helps, and all the actual missions are brilliant