NHL: Have handled the concussion issue better, games are just really fun to watch, great parity, keeps smaller markets engaged and able to compete. Outside of the Commish, there's really nothing bad about this league. The speed of the game is crazy, the skill is getting better, and it's still physical while eliminating the vast majority of unnecessary head shots. Wish it had a little more personality though.
NBA: I dislike their style over substance at times, but they really are entertaining. Would be so much moreso without the Warriors being the entire Western All Star team. But it's an uber-athletic game and fun to watch and their stars are well-marketed.
MLB: Games drag on, but I like the rivalries and it's pretty cheap to go to games. I wish they would get out of their own way with the Unwritten Rules though. The playoffs are more fun to watch than the regular season, which is not always the case in sports.
NFL: I honestly don't watch the NFL anymore. They've eliminated defense, they're oversaturated, overmarketed, and underwhelming in terms of entertainment. Too many flags, very little (if any) innovation from coaches at the highest level of their sport. The evolution of QBs and WR would be impressive if the rules allowed defenses to actually play them because some of the throws/catches are incredible, but you undercut these plays when you're not allowed to breathe on either participant. It's kinda like if baseball banned gloves and were like "Look at all these triples!" No one knows what a catch is anymore. No one knows where you can hit the QB anymore. These are some of the most fundamentally basic things about the sport, and even die-hard fans of the sport are completely confused and frustrated by them. I'll watch 80's-2005 games all day long, but you couldn't pay me to watch whatever this is.