Line changes are fun once in a while, but the pace of the game really slows down. With the online stats, and +/- hack, it's a very interesting way to play from a stats-nerd perspective.
When it comes to manual goalie, it's completely different using manual goalie vs the computer, and manual goalie vs a real person. I would almost never use manual goalie vs the computer (other than to negate icings), you're much better off playing defense. But when you're playing against a real person, you will quickly learn the times when if you don't take over the goalie, the other guy gets an automatic goal. For example, the "crease cut" move, the AI goalie will flop on the ice every time, but if you take control of the goalie, it's an easy stop; another case is the slapshot from the slot, which 90% of the team beats the goalie, but you can just take the goalie and shift him over a little and block it.
I have only encountered 2 guys who were upset about being checked after the whistle. I consider it part of the fun, its own little mini-game. The exception is if I'm winning by a lot of goals, I might stop checking after the whistle so as not to appear to be rubbing it in. One time, I had a guy get upset because I checked his player into the penalty box during a penalty animation, which caused him to have to wait 2 extra seconds for the player to get up out of the box, then go back into the box to serve the penalty; I found that a pretty ridiculous thing to get upset about. One of the rarest and most treasured feats in the game is to check all 5 celebrating players after getting scored on.