Artistic License - Pharmacology: Vicodin is prescribed more than 100 million times each year in the US to treat moderate pain (like arthritis) and to treat coughs. Even in long-term users, Vicodin withdrawal is usually characterized by symptoms no worse than lack of appetite, mild nausea, irritability, anxiety, and restlessness. The withdrawal symptoms House usually displays (vomiting, insomnia, sweats and chills, depression, mood swings) are what would typically characterize frequent diacetylmorphine (heroin) abuse. Vicodin is also not known to cause disassociative disorder (conditions that involve disruptions or breakdowns of memory, awareness, identity and/or perception). That being said, the plotline of season 3 suggests that House is taking a fair amount more than a typical patient.
Methadone is an long-acting synthetic opioid agonist that, like any opiate, causes euphoria. It is prescribed to treat pain in opioid-dependent patients as well as addiction in heavy users of high-potency opiates. Because methadone is many, many times more powerful than hydrocodone (it's listed as a greater than 2 to 1 conversion compared to baseline (morphine), whereas hydrocodone has no consensus but is known to be less than 0.33 to 1), methadone is never used to treat even the heaviest Vicodin abuse. That would be like prescribing 99 Bananas to someone addicted to wine... no matter how much wine they drink, taking shots of vodka just isn't going to improve the situation. There are also other opioids which are more effective at managing pain, so if they specifically wanted to curb House's vicodin use (likely, due to the risk acetaminophen poses to the liver) they would step up to oxycontin (0.33 to 1) or ms-contin (1 to 1).
Methadone is used to prevent heroin abuse, mainly because there are a limited number of opioid receptors in the brain and methadone fills them up (preventing heroin's effects from being felt), and because it delays the onset of withdrawal symptoms for many hours, thus reducing both the cravings and incentives for abuse in patients who take it correctly. Methadone users continue to experience physical dependence, but after the addiction is under control, the dose can be lowered to reduce dependence.
Fake American: Hugh Laurie as Dr. House. His American accent is one of the better examples, though the way he pronounces some words can give it away. Strangely, he keeps the accent even when he's screwed up lines, as can be seen in the outtakes. When executive producer Bryan Singer saw Hugh Laurie's audition tape, he turned to the casting department and said, "See? This is an American actor!" The casting department had to correct him.
Lampshaded in season one when House calls a doctor in the early hours of the morning. When asked to explain why he is calling at such an early hour he "puts on" an English accent and pretends he was calling from the UK and hadn't considered the time difference. For this scene Hugh Laurie is of course putting on the silly voice he used for oddball sketch comedy in the '80s.
Like most fake-American accents, Laurie uses a "gruff voice" as a cover-up in order to fake an American accent over his British — à la Bob Hoskins in Who Framed Roger Rabbit, and John Mahoney on Frasier; not coincidentally, they're all playing stereotypical "tough American detectives" who supposedly all speak in gruff Midwestern dialects.
Michelle Trachtenberg (best known for playing Dawn Summers on Buffy the Vampire Slayer) guest starred in the episode Safe. She's admitted to having a crush on Hugh Laurie and wrote "I love you" on her inner thigh before a scene where he'd seen it. We might be getting to Stalker with a Crush territory with that.
House: Got all of my starters back plus a couple of free agents. I feel like Mike Tomlin. (looks at Foreman) Probably not as much as you do, but you get the idea.
Superlative Dubbing: The Spaniard dub is one of the best in an American TV series in recent memory. Specially House's voice actor, Luis Pórcar. Eventhough his voice doesn't sound like Hugh Laurie's (although it fits him, nonetheless), his performance is so good that even a lot of people that are normally against dubs watch each episode twice: once in English and once in Spanish. All just to enjoy both voices.
This is specially surprising considering that, due to the huge popularity the series has in Spain, they have to dub every chapter insanely quickly in order to not lose TV audience when they broadcast it.
A physical deformity or handicap was always part of the character design for House, but early versions of the show put him in a wheelchair rather than giving him a bum leg; this was thrown out because it limited the character but showed up in the episode "Needle In A Haystack", where House tries to go a week in a wheelchair as a bet. In another early character design, he had a giant scar on his face.
"Chasing Zebras" was an early working title for the show, after the common med-school saying "if you hear hoofbeats, you think horses, not zebras" (i.e. a simple, mundane explanation for a group of symptoms is likelier to be right than an exotic or complicated one).