3. Bad Gunplay
In the rest of the game your battles are against squads of guys. Your goal is to pick them off or sneak by them. These encounters are interesting and full of choices. The boss fights are one-dimensional, obvious, and boring. You do a tiny bit of damage to the bad guy. He does massive damage to you. Chip away at his health bar without taking more than one consecutive hit, or its game over. If you're a stealth-focused character, or if you've sunk your skill points into the combat abilities that the bosses are immune to, then you are going to spend a good bit of time at the loading screen.
4. Bad Combat Taunts
You're playing a sports game, like sports people do. Suddenly you destroy the morale of the other team with a single, well-timed cutting remark. Maybe something about someone's mother. Your teammates high-five you. Everyone laughs. Then five seconds later, you say the exact same thing. People stare at you, because you just repeated yourself. Now you say it again, mechanically. Now people are laughing at you. You're not clever. You're a dumbass with no common sense, and you ran out of material two taunts ago.
Now imagine the same scenario, except the original remark wasn't clever at all. It was the equivalent of "You guys suck!" That's every boss fight in Deus Ex: Human Repetition.
I don't know why game designers do this. Boss fights have been like this for years, and it's always a silly immersion-breaker to have your foe repeat himself like an iPod Shuffle with only two songs. It's particularly bad in this case, because the repeat interval is ridiculous. In one fight, I actually had the boss interrupt himself, so he could repeat the taunt he was already saying.
In the third fight, the taunts are actually self-refuting nonsense. I was standing in the open, drilling the idiot with minigun fire when he turned invisible and slipped away. Then he taunted me for "hiding" from him.
The taunts are on a timer and have nothing to do with what is happening on the battlefield. If you blast your foe in the kisser with the shotgun, his taunt will continue without so much as a stutter. It's also confusing, since it feels like you aren't doing any damage at all.
5. Bad Storytelling
This game is filled with vibrant characters. Everyone has an agenda to pursue, or a philosophy to share. Some are allies, some are rivals, and a few are a bit of both. Each of them gives us a view of the larger world by representing some group or way of thinking.
In contrast, the boss villains are flat clichés. They are archetypes and plot devices. They tell us nothing about the world or about the foe you're fighting, or about why they would die to oppose you. They are just generic henchmen thugs with super powers.
It's a shame Eidos decided to outsource the boss fights, and even worse that they apparently outsourced them to people who had no understanding of the rest of the game. Human Revolution is otherwise excellent, merging the robust depth of those late 90's shooters with the polish and production values of today. But whenever we look back at this jewel of a game, our praise will always be followed by an asterisk, a disclaimer, an unspoken lament for the game that could have been. Yeah, the game was mostly awesome. Too bad about those boss fights.