Complete Monster: Bernard Loredo is the thuggish Commandant of Flotsam who runs the town like his own personal fiefdom, running criminal enterprises and instituting racist policies against nonhumans. When Iorveth's men kill some of Loredo's, Loredo sends out his soldiers to incite riots against the innocent, nonhuman residents of Flotsam, leading to butchery and mass murder of nonhumans in the streets. Eventually, Loredo has the Scoia'tael members and their sympathizers rounded up on a prison barge which he plans on sinking instead of delivering to prison. If Geralt sides with Roche and kills Loredo, the Witcher will discover, despite Loredo's hatred of nonhumans, the Commandant imprisoned, beat and raped an elven woman for a year, eventually impregnating her. The woman was so traumatized by the events that immediately after giving birth she killed herself. Should Geralt choose to side with Iorveth and hijack the prison barge, Loredo responds by spitefully trying to burn down a building full of elven women. If Geralt chooses to save the women rather than kill Loredo, Loredo will succeed in turning traitor and selling out Flotsam to the kingdom of Kaedwen.
Fridge Brilliance: Geralt and Letho are in the end Not So Different; both Witchers, but unusually compassionate people whom have been conscripted into the service of a Monarch whether they like it or not, and both of them are motivated beyond self-interest to willingly stay in their service. The contrast is that Geralt doesn't want to bring unnecessary conflict to himself and his friends while Letho is trying to create a safe haven for his own kind.
Game Breaker: The one from the previous game, the Igni Sign had been nerfed, but Aard and Quen (and even Axii to a point) can easily reach this level.
Bombs. You can make a load of them and just chuck them out like candies. Works especially well on bosses, even better if you brought the skill that multiplies the damage dealt by 100%, and it can be taken even further with a special torso armor. And the fact that the basic(Grapeshot) requires just need two most common alchemy ingredients, has a good range and damage plus can hit a lot of targets at once just further drives the point home.
Heck, Igni still counts too, you just have to invest in it. With maxed spell damage enhancements and skills, hammering the sign key to machine gun out fireballs will rapidly drain the health of anything not immune to fire. If your enemy is one of the few which is fireproof, blast em' anyway- a single blast from Igni will force them into a few seconds of flinch animation, dropping their guard.
Golden Ending: While no clear "good ending" is even possible, there are few better choices and their quality of "good" itself can vary. Of course this can also change with the release of The Witcher 3. Almost all of them are counter-intuitive.
Saving Triss is always better for the world in the long run than securing any other person, no matter which side was picked. This is the only path leading to reinstitution of Council and Chapter of Mages, thus restoring the balance of powers and securing cooperation between mages and kings, instead of rampant powerplay in style of Philippa's Lodge. Not saving Triss personally leads to pogroms of mages and magnifies the chaos in the North even further, additionally robbing kings of their (often well-meaning and trusted) advisors and high courtiers and sowing distrust among Nordlings. This choice makes much more sense after reading the saga and playing the games - almost all the mess in the whole franchise was caused after the Council and Chapter were disbanded in the wake of Thanedd coup.
Giving Ana´s to Radovid is better than sending her to John Natalis - as Roche points out, better to be a protectorate of Redania than succumb into a civil war and doing so right when Nilfgaard started it's invasion. The civil war could be possible even worse than partiation of Temeria into baronies.
Stomaching what Henselt did and letting him alive means that Kaedwen will hold fast rather than disintegrating like Temeria and Aedirn did. If Kaedwen falls into interregum anarchy, this leaves Redania as the only country in the North, while lands of Kaedwen, Aedirn, Temeria and (unmentioned in the game) Lyria will fall into hands of Nilfgaard without any trouble, thus leading to situation where Redania will fall eventually, stripped of any potential allies and being surrounded by Nilfgaard. If Henselt remains in power, Kaedwen takes Aedirn and Pontar Valley, but the country is still there, with it's ruler in charge.
It's better to let Prince Stennis live rather than allow the peasants to lynch him, even if he possibly had a hand in poisoning Saskia. Without him, Lower Aedirn descends into anarchy, making it more ripe for Nilfgaard to take.
Most controversial choice requires from player to intentionally weaken Northern Kingdoms and thus shortening the inevetable war with Nilfgaard into a series of mopping operations rather than all-out war.
Indeed, all of it does change with The Witcher 3, which pretty much nullifies everything described above. The mage pogroms are slightly postponed at best, Nilfgaard invades and Temeria is toast no matter what. Essentially the only decision that ends up doing any kind of difference is helping Saskia on Iorveth's path. It's the only ending where at least all of your major allies can be well at the end of the game.
The deciding factor for most players killing or sparing Henselt is probably whether or not they can stomach the thought of him living after raping Ves and gloating about it right in front of Geralt and Roche.
The above doubles as one for Roche. Henselt just killed all of his men and raped Ves. His purpose through the game was to track down the kingslayer, only to be willing (or for some, successfully) to become one himself.
Depending on the playthrough, Loredo has a few potential moments where he may have crossed it, exactly when depends on the player. The first can be when he arranges for riots and mass murder of innocent nonhumans in Flotsam. The second can be when Geralt sides with Roche and discovers Loredo's kept an elven woman as a Sex Slave for a year. The third can be when Geralt sides with Iorveth and Loredo personally sets fire to a house full of innocent elven women just to spite the Witcher.
Polished Port: Barring lower graphical fidelity and visuals, the Xbox 360 port made many significant improvements over the PC original. The controls were revised for the controller and introduced a new tutorial that thoroughly explained the mechanics of the game. The port also tweaked the game's difficulty curve of the prologue mission, meaning that players would no longer have to worry about being stuck as a result of attacks being interrupted mid-swing by an enemy hitting Geralt from the side. As if that wasn't enough, people who purchased the port received a game guide, a world map, concept art, interviews, a soundtrack, four hours of additional gameplay, and a code to redeem a separate PC copy from GOG.
Stripperiffic: Saskia's armor shows a lot of cleavage over providing protection to her heart and lungs. Potential Fridge Brilliance however, as she doesn't need the protection anyway, given that she's actually a dragon; her ability to heal is epitomised by quickly recovering from impalement at the end of chapter 3. Also, the sexy look could help sway the peasantry to her cause (as evidenced by one stolen dream you recover being a peasant having a sexual fantasy of Saskia).