Which parts of a new character should the player have to explain in some detail in the backstory? Why or why not?

Unusually high or low attributes?
Unusually high or low skills?
Edges/flaws?
Awakening?
High resources and/or lifestyle and/or spells?
Contacts? only buddies and ffl or all of them?
Unusual pieces of equipment?
Cyberware? - or even backstory replacement of cyberware?
Bioware?
Other?

(I had been intending to do this as a poll, but there are too many options and combinations of options to make it practical unless there were some way to set up a given poll to allow more than one answer: not generally desirable, not asked, not done.)

All of it.

That was easy, no?

Now, I don't mean that they need to explain each and every little detail, but the background should contain enough detail for plausible extrapolation to be able to take place.

Easy perhaps - and yet I remember this huge argument in the old forums over how explaining the existence of an edge or flaw undermined its value: it just was, and that's how it was supposed to be. So perhaps not everyone sees the same need to explain the same things?

I don't require much background info on why/how you got stuff. In our games, we kinda assume you're not new to the shadows, and if you'd have a reason to get it post-intro, you'd have had a reason to get it pre-intro. I do ASK for (but not require) background to explain the why for a piece, but it's not for my approval, it's only to help them get a feel for their character. If someone tells me he got Wired 3 to bake cookies faster, we'll work on that, until the background meets the Wired 3. People make characters they'll enjoy playing, I help them make the background to fit the character they'll enjoy playing purely so they have a better "feel" for their character. I don't limit access to anything at all (except the prohibitted Mnemonic Enhancers) purely because I like playing with people who LOVE their characters to the point of not playing in a manner that gets them killed.

I think that's how it should be, fit the background to the character, not the other way around. (The SR3 book even suggests that actually, and with good reason IMHO).

Sphynx

I believe the backstory helps both the player and GM in actual roleplaying.

My character is a samurai, he used to be a Lone Star cop who was kicked off the force for getting revenge on the Humanis Policlub cell that killed his troll sister. Though he managed to get revenge for her slaying, he himself feels very guilty because he rarely spoke to her after she trolled-out (they were estranged for 20 odd-years), - essentially he was prejudiced against metahumans.

When she (his troll sister) was caught tortured and killed in some Seattle back alley by a militant HP cell, he realized that he was an idiot. He arranged for a transfer to an elite wing of Lonestar and got some cyberware from them, then armed to the teeth, and after much research he broke into the HP cell while they were meeting and slew them all.

He was so well thought of in the force, that he avoided prosecution (self defense) but had to resign. (He has 3 Lone Star cops as level 1 or 2 contacts as well as purchasing Lone Star itself as a level 2 contact which is there only to reflect the goodwill that other cops still have for him. "He was a good cop, and if anyone else messed with my family I'd have done the same".

This helped me get knowledge skills on humanis policlub and cyberware. Explains why I have cyberware, why I have the flaw Intolerance to Racial Prejudice (very touchy because he feels guilty about how he treated his sister), background trouble (he got press after the incident - some people love him for killing the HP cell, others hate him for the same reason)..and most of all how he got his street name - from the headlines of the article reporting the incident "Lone Star Cop Goes Nova"- hence his street name is Nova.

I enjoy my character much more because he is cohesive, and I think it helps my GM (Velocity) in finding new and subtle ways to harrass me.

Nova

I tend to think a characters backstory should explain most of that list, because the reasons for all of those things help define the characters personality, which is the most important function of the backstory to me as both a player and a GM. The answers to a lot of those questions also help give the GM material to reach the character on a more personal level than trying to do things just based off of numbers, and help predict how a character will react to certain situations, which is incredibly important to me as a GM, as I find runs tend to become far to bland very quickly without some personal hooks to individual characters.

Maybe you don't need to have a written explanation for everything at character creation, but you sure as hell should be able to justify it, or at least account for it. Even if it's 'My character was born freakishly smart/ color blind/ rich/ immoral.'

I endorse blue booking. That is, keep a running development of your character's background as the issues come up. For example, maybe you didn't list all your family members when you made the guy up. One game session, for dramatic value, the GM interrupts a gunfight with a character's phone call...

player: "Hello? Who is it?"
GM (in old lady voice) "You haven't called me all week. Are you sick? What's that racket?"
player then marks in his blue book: 'Has pestering mother.'

Id say theyve got to explain anything un-ordinary or exceptional, plus generic biographical information. Born, school, where you got cyber, etc. If youve got quickness 6 cause youre agile fine, no need to explain that as long as aglie fits with the rest. Allergy? Yea you could be born with that. But Flashabcks? Now thats gonna take some explaining. Etc etc etc.