The major changes I noticed between the two editions are as follows:
Limits prevent characters from being overly min-maxed. Each of them is centered around an attribute that is typically dumped in normal characters; the most important attribute for the physical limit, for instance, is Strength, though other attributes weigh in they have the same impact as Strength does alone. This means that you can't build a "never gonna fight close quarters" build and just dump strength and expect to do well in other physical areas, encouraging a well-rounded thing.
Mystic adepts get a huge buff. I'm not exactly sure that this is a bad thing; they still can't astrally project, but they get the powers of both mages (other than astral) and phys-ads pretty nicely. Were they still using the BP system, this would be a flaw, and I'm not sure about allowing them as the third pick on your priority system, but I think the reason that people are upset is because they don't astrally project as much as they should when not a mystic adept.
The priority system really makes things a lot better. It prevents some of the worst cheesing during character creation (don't get me wrong-it's still possible, but you have to know what you're doing and make some sacrifices).
Combat's been changed rather heavily on the bookkeeping, but not so much the execution. The Accuracy limit keeps pistols from killing Great Dragons, which is a nice touch, but also discourages just dumping into the newly increased skills and maxing them out right away. It also makes smartlinks a more tangible advantage, as do laser sights. Armor is now a single rating for stun and physical, which makes it a lot easier for new players to understand, and, in my opinion, more realistic.
Hacking's a lot better. Mind you; the wireless thing contains some logic holes and gimmicks with the new benefits it gives stuff like cyberware or laser sights, but hackers can enjoy a target rich playground with new rules for hacking that make prepping a hacker 90% easier and playing one about 50% easier; GM'ing hacking also became a lot easier. In addition, some of the more broken technomancer stuff has been revised so you now have a reason to play a decker instead of a technomancer every single time.
All in all, it's faster and more streamlined. If you want my "reviewer" version, you can check it out on my blog, but I've said pretty much everything I said there here, only without the sales pitches.
The new errata answers the question clearly:
Change to following sentence: “You get a free Power Point whenever you
increase your Magic attribute, and you can gain a Power Point through
Initiation (p. 324) instead of gaining a metamagic.” To: “If you’re an
adept, you get a free Power Point whenever you increase your Magic
attribute (though this doesn’t apply to mystic adepts, and you can
gain a Power Point through Initiation (p. 324) instead of gaining a
metamagic.”
Therefore, further power points do not require spending of karma, but do require to forfeit a metamagic when initiating, and can only be acquired by initiation.
Best Answer
In short, a SIN is character's general physical information (i.e. DOB, metatype, etc.) and some other data (criminal records and whatnot) about them stored in a database run by a nation or corporation. In addition to this use of the information, SINs are often used for advertisers and financial transactions, by being linked to a credstick or commlink, allowing for their owners to be tracked but also experience a highly customized lifestyle that theoretically improves their quality of life.
Pages 366-368 have the rules for the mechanical functions of a (fake) SIN; essentially, your SIN is theoretically broadcast from your commlink at all times, but you can set it up to use any of your SINs, or even just not to broadcast (though this gets you in trouble in the uber-secure areas). As a general rule, unless the player has already presented another SIN, they may choose the one they want to use when questioned/checked/otherwise harassed about handing over a SIN. The quality of a SIN determines how "real" it is without, of course, being your normal SIN, unless you're SINless.
The point of the SINner negative quality is for three reasons:
The first of these is pretty simple; it's theoretically possible to look up a person's SIN using biometrics, so if you get taken in for doing something, you need to consider the fact that you'll want a fake to present before they find your real identity. Usually, unless it's an obvious fake, that sticks your (hopefully clean) fake SIN with the criminal record, at which point you can ditch it and get another.
Taxes, of course, are one of the two things which are certain in life, and one of the upsides of not having an official identity is that you also do not wind up paying taxes; some of this is offset by the fact that the SINless may have to pay for public services a SINner can access free, but as a general rule a 400 nuyen bus pass is still cheaper than 3600 a year in taxes.
And scrutiny is the largest one. The SINless distrust SINners, especially corporate types; it's one thing if you're born into the squalor of a ditch in the UCAS but your parent(s) happened to have a SIN or two between them and got you one, but if you've got a high-level corp SIN you're essentially looking at being set for life with a silver spoon, unless you do something horrible. Also, while it's not technically scrutiny, people with corporate SINs often find themselves "scouted" by rival corporations or even other branches of their own corporations, and extractions can be dangerous for everyone involved.