These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The Rumbler is objectively powerful, with its limitless mini-turrets and rockets, but much like a rocket turret, it's more or less completely shut out by the Telekinesis plasmid.
The King Mook Subject Omega, a.k.a. Alpha Sinclair, only has the benefit of a longer health bar, a triviality to anyone who's maxed out on their research of the Alpha series. It's not an exaggeration to state that a single shot from Incinerate 3 can potentially kill him on the highest difficulty, in tandem with the hazards in the room (though this is unlikely). Even conventionally, the battle won't last much longer than 30 seconds. Heck, since you have the Summon Eleanor plasmid by this point, she can kill him for you before you ever enter the room. The dual Big Sisters fought one level earlier are the real Climax Boss of the game.
Complacent Gaming Syndrome: Similar to the the wrench and electrobolt combo from BioShock (which is still viable here, just not as broken), we have the upgraded drill and Winter Blast combo (although some people opt to use Insect Swarm instead). Oh, and go and complain afterward.
Crazy Awesome: Gilbert Alexander was pretty awesome before going mad (leaving behind messages and robots allies to help whoever found him kill him, knowing he was about to suffer With Great Power Comes Great Insanity) but he still pulls off some cool stuff afterwards (such as singing the Fontaine Futuristics jingle at the top of his voice to prevent you from accessing a voice activated door lock, or hosting a live demonstration of Fontaine Futuristics products...that try to kill you.)
Demonic Spiders: Minerva's Den has the Lancer Daddies. Their lasers are highly accurate and fire extremely fast, they chase you relentlessly while still shooting you, and you don't get the option of researching them to be weaker. You only have to fight six, but they're the six toughest fights in the DLC. To stress how powerful they are, a one-on-one between them and a Big Sister will usually result in the Sister dying.
Disappointing Last Level: Inner Persephone is this for some. While the level is still fun to explore, there isn't any real final boss unless you count Sinclair as Subject Omega, who fights like any other Alpha Series but with a longer health bar, the summon Eleanor plasmid makes every enemy encounter trivial, and there isn't any other encounter with Sofia Lamb beyond her shouting more of her philosophy at you over your radio.
Ensemble Darkhorse: Alex the Great is very popular among the fanbase. Owing in no small part because of his memorable dialogue and being batshit insane, even when placed among Rapture's inhabitants.
Despite being the star of Something In the Sea, Mark Meltzer wasn't originally going to be in the game. However, the fandom's intense love for him motivated 2K to add him into BioShock 2 and wrap up his story.
Evil Is Sexy: Sofia Lamb gets a lot of attention regarding her looks from the 2K forums. As you can imagine her strict authoritarian personality also doesn't hurt.
Fanon: Subject Delta's suit-machinery includes a built-in can opener.
Foe Yay: The Brute gets downright flirtatious with Delta under the influence of the Hypnotize plasmid.
Much like the first game, the One-Two Punch of Electrobolt and Drill will reliably kill most anything, though it comes at a much higher EVE cost until you've got some tonics to negate some of that.
The third tier of Decoy, which not only distracts foes but damages them and heals you while doing it.
A fully-upgraded Rivet Gun, whose flaming shots allow you to juggle-stun enemies when paired with Electrobolt.
The Fountain of Youth tonic causes you to regenerate health and EVE while standing in either a pool of water or under any significant leak. There's pretty much nowhere in the game you won't find one of these, so it becomes trivial to just stand in one and let fly with plasmids, safe in the knowledge that you'll never need a hypo because the regeneration will kick in before the automatic injection.
You've also got the 'Cure All' Gene Tonic, which makes each Health Station refill your EVE meter fully. This lets you pull of tricks like setting up dozens of traps near a Big Daddy or a corpse your Little Sister needs harvested, then hurting yourself slightly and coming back to the Health Station to repeat the process for a pittance.
Using the second tier Security Command and Hypnotize in conjunction. The former lets you summon security bots, but on your side and the latter lets you brainwash a normally hostile splicer to assist you; summoning two bots and hypnotizing a spider or leatherhead splicer allows you to easily overwhelm opponents whose only real advantage otherwise was strength in numbers. You can even up the ante in numerous ways: the handyman, deadly machines, and hardy machines tonics make it easier to keep bots around and makes them more powerful, while the third tiers of Security Command and Hypnotize let you summon even stronger bots and hypnotize anything but bots and Big Sisters. By the end of the game you can also include Summon Eleanor into the mix, and when you have two elite security bots, an alpha series, and a Big Sister helping you, your enemies become so incredibly outmatched you barely have to fire shots at all. The only real downside to this strategy is the high EVE cost (which is easily remedied with the aforementioned Cure All tonic) and the fact that having so many entities around makes it easy to to get caught in friendly fire.
The Freeze plasmid is also overpowered if kind of repetitive to use. It's relatively cheap to use, works on ALL enemies, and health gets drained faster than it did in BioShock. Combine with Security Bots and/or Decoy to distract other enemies and you can essentially knock off enemies one by one staying completely safe.
The Drill Master Tonic turns off all your ranged weapons, but who needs them when plasmids cost practically nothing to use and there's a whole range of melee-buffing tonics to employ? Not to mention the right plasmids can almost completely cover your ranged deficiency. It turns you into a plasmid throwing melee god who will run out of health long before running out of EVE.
Insect Swarm paralyzes enemies while you kill them, for much longer than Electro Bolt (but without the damage bonus). This is especially helpful in Minerva's Den, where you're confronted with the tougher splicers much earlier on.
One that is fully intentional is the Summon Eleanor Plasmid. You receive it during the final mission of the game, and it turns what might have been a Disappointing Last Level into a fun two-person romp through hordes of splicers who are completely unable to fight off the combined force of a Big Daddy and a Big Sister.
Hilarious in Hindsight: Or perhaps Hypocritical Humor. Near the end of the game, Sofia rants about you "stealing" her daughter from her and about Eleanor's "ungratefulness" to her loving mother. However, some of the earliest tape diaries you found of Sofia reveal just how little she actually cares for Eleanor as anything other then her tool — Eleanor was never really her daughter in anything except terms of shared genetics. One even says she had "as little to do as possible" with her birth, implying that Sofia may have actually created Eleanor through IVF and had a surrogate carry her to term, simply because having to deal with pregnancy itself would be too much of a hindrance to her plans. The hypocrisy about stealing Eleanor from her takes on another level when you remember that she herself has been stealing little girls from their families on the surface.
Internet Backdraft: After having waited for weeks, the announcement that the single player DLC, particularly Minerva's Den, wouldn't come out for PC anyway was taken poorly by the fanbase. Fortunately, 2K listened and immediately resumed work on it. As of October 2013, both the Protector's Trials and Minerva's Den are available on Steam.
Sofia Lamb in the opening cutscene, where she forces you to commit suicide in front of poor Eleanor, your bonded Little Sister and her own daughter. The woman makes new Moral Event Horizons with each appearance just to cross them.Though as you'll learn during the game she appears to have crossed the line long ago. She'd have you believe that she's going to initiate a new Utopian era for mankind, but thinks nothing of choice and subjects her closest comrades to half-truths and horrific fates. She leaves Gilbert Alexander to live forever as an abomination, desperate for a taste of ADAM, Mark Meltzer is reunited with his daughter and twisted into a Big Daddy, and Augustus Sinclair becomes an Alpha Series Big Daddy, and she snidely remarks that the two of you should be the last, as you fight to the death. Her intended fate for her daughter, Eleanor, is to become the base in a gestalt of Rapture's greatest minds, creating a 'Utopian' being with the perfect outlook on the world, yet crushing Eleanor's own personality and conscience in the process. She's potentially much worse than Ryan given she's the "believes-every-word-she-says" type of crazy. She genuinely, lovingly accepts a few thousand deaths for the common good.
Eleanor Lamb can potentially cross this in two of the game's darker endings, killing her mother Sofia in both. In the games worst ending she takes her fathers memories with the intent of using them to help her "take on the world" as it were. Though very much averted in the other two good endings.
Finding Mark Meltzer. Those who followed the ARG know this moment. In fact, an entire thread entitled "How DARE you 2K..." was started on the forums for players to chronicle their reactions to the discovery, which ranged from stunned incoherence to ragequitting the entire game. Many felt more strongly about this event than the actual ending of the game.
The fate of Augustus Sinclair. As a punishment for helping through your journey, he gets turned into a Big Daddy. To pile on the Punch, he struggles through his mind control to give you more information, and asks that you end his life in return.
Most Annoying Sound / Welcome to Corneria: The splicer dialogue, after a while. Since they have only so many lines of dialogue and they constantly spawn to attack you, in some levels you end up hearing the same lines repeated endlessly. The worst of these is arguably Siren Alley, where the sheer number of Baby Jane splicers there mean you'll hear screams of "Not again!" and "Hack!" almost every minute.
The Untwist: Lets just say the game doesn't feel the need to succumb to Shyamalan Syndrome, and never feels pressured to throw in a mindblowing plot twist towards the end just because the first game had one. Also josses all the Epileptic Trees speculating that Sinclair was working for Fontaine, or at least would pull a Fontaine. Given the similarly spectacular twist in BioShock Infinite and even the small surprise in Minerva's Den, this makes BioShock 2 even more of an odd man out.
They Wasted A Perfectly Good Game Mechanic: The U-Invents don't return. Especially unfortunate since the Gathering process invokes much more use of things such as Trap Bolts (or Trap Spears, in this case).
What an Idiot: Wadsworth, an opportunist only mentioned in the loading quotes for the multiplayer. He was under the impression that the Sisters could be tempted with another job. Keep in mind that there were several PA announcements (one of which is another loading quote) saying "Don't mess with the Little Sisters, or the Daddies will hurt you."
You'd Expect: Him to see the warning for what it is and leave them alone.
Instead: "I wonder what I can use to offer the job? Kids like candy, right?" His unrecognizable corpse is found with signs of Big Daddy damage on it, and he is reported to have attempted giving a Little Sister a lollipop when the Daddy noticed and flipped out.
Grace Holloway's life reads like a checklist of misery and misfortunes.
Gil Alexander, believe it or not, at least before he became Alex The Great. His audio diaries show his admiration for Sofia Lamb, her intelligence and her philosophy, but there are subtle hints that he's also in love with her, even though he knows she doesn't reciprocate. He's perfectly willing to volunteer to be made into a monster for her, and when she abandons him as a failed experiment, he never shows any resentment, only a regret that he failed her. There's even some fan speculation that he might be Eleanor's biological father.
Poor Eleanor Lamb. She spent her entire life being raised by an emotionally distant mother who fully expected Eleanor to further her own ideals by getting turned into a Hive Mind that would only serve society. Depending on the player's moral choices, she can also become a Woobie, Destroyer of Worlds.
C.M. Porter from Minerva's Den. He couldn't handle the death of his wife, so he spent all of his time uploading as many memories of her as possible into Rapture's central computer in the hopes of recreating her. And when he finally finished he was horrified by what he had done, because it wasn't really his wife, and had the computer shut the whole thing down. Then he was captured and turned into a Big Daddy.
And then, there's many of the multiplayer characters, leading to much Fridge Horror when you realize what their ramblings actually mean:
Why does Barbara seem to think the Little Sisters she captures are her children? Because her daughter was turned into one, and what's more, her abductors came in the guise of ADAM merchants - ADAM which she wanted to protect her daughter with.
Jacob acts as though it's just another day at his job because he's been so browbeaten by the generally miserable working conditions that he's convinced himself he's on a work strike against them.
Buck seems like an unsympathetic jerk until you discover that he just wanted to find his wife, and that his last encounter with her was him discovering that she had joined Atlas-while she nearly murdered him. No wonder he's bitter.
Shuresh Seti may be Crazy Awesomebut keep in mind he may be telepathic...and that he's perfectly effing sane. Now, imagine what it would be like for you to be privvy to the thoughts of Splicers.
Blanche is a good example of how Rapture's cosmetic business can destroy a person. She just didn't want to be lonely...
Let's face it, Louis is a violent jackass but he knows full damn well how Big Daddies are created and he's actually scared out of his mind of being turned into one-he actually thinks the enemy players are trying to send him to processing.