The flat number of bonus actions does have me worried. One of the annoying things with earlier editions is that you HAD to have at least 2 actions per round or you were dead, wet meat on a stick.
People talk about the munchkinizing of D&D, but it is a pale shadow (hah!) of Shadowrun, at least up through 3e. If you don't bend the rules until they're screaming for mercy, you won't even survive to meet Mr. Johnson, much less make it out onto the mission...
Init in 2e was really wacked, with the faster people going first multiple times. 3e fixed that a bit in everyone went first once, but the wired out the wang people still aways went first. In SR4 initiative order is determined by a roll and is independant of the IP (initiative passes), so even a 1 IP character might act before a 4 IP character, and Edge can buy you a chance to go first as well.
In 4e you are still mostly hosed in combat if you have no way to bump up to 2 IP, but that is very easy augmentation via chrome, bioware (engineered flesh, first introduced late 2e I think), or magic. Or if you go into Virtual Reality and to control a drone or do matrix work you then have 2 IP or 3 IP. Or with the new Edge attribute (that more or less replaces the function of the old Combat Pool and Karma Pool) because one of it's uses is to allow you to act during a single Init Pass you normally wouldn't be able to.
4 IP is the obsolute top end ever, instead of the 6 passes IIRC you could roll in SR3, so that keeps it a bit more sane.
As for obsolete books, Shadowrun has a definate divide in their books. The 4 or 5 core crunch books are a rebuy, which isn't too bad after 7 years of SR3. The fluff books are however completely reusable. For example Shadows of Asia which was the last geographic setting book to come how is over 200 pages long. Of outside a multipage appendix table rating the Matrix hosts, maybe 3 pages mention any actual game mechnics stuff at all. Virtually total reuse.
No, the reason for stink is mostly along the tack of "the rules have to onerously hard to learn, or there is no way to provide a mental challenge, and you are just pandering the the unwashed masses of stupid people". Plus there some folks hung up on "realism". They want to play a simulation. Unfortunately SR3 wasn't really a simulation either. The rules were just so convuluted and complex that it simulated a simulation. The results were in fact quite bizzare. The Rigger 3 book, if you were so unlucky as to try use it verbatum, was an extreme example of that.
So "realism" is mostly a code word in one of two sense. "Realism" for players means rules so complex that they are a pain in the ass. "Realism" for characters means trying to do anything is so hard that it is a pain in the ass. :imsorry:
On the new wireless Matrix, basically what it does is has riggers and deckers using the same basic tech and exist in the same world, and even using the same initiative. It also gives a lot more mobility to deckers right out of the box. Though there were ways by canon in SR3 you could have a bit more mobility, deckers were pretty much confined to their mom's basement. SR4 got them out of the basement.
That actually sums up the biggest change from SR3 to SR4. It is now one game instead of 3 or 4 games that sort of work with each other but not quite. Even the dice system, while "new" isn't entirely foreign to the old dice system. SR3 had so damn many dicing variations that were wildly different from each other. They just picked one, fixed TN, and used that as a theme throughout.
EDIT
On munkinism. One thing to keep in mind that the power curve for characters is a lot flatter on advancement than D&D. So while a starting character is very roughly speaking about 5th level, you really only scale up to about 10th level equivalent in practice. Also it isn't designed to keep the power of the character kinda even through out the character, you have more flexibility in where you allocate your strength. So right out of the gate a PC can pull off some really amazing feats, but the have to specialize to the point where they make the character less viable in a well rounded game. So munkin style players tend to really stand out. All the more reason not to game with people that aren't on the same page as you in what kind of game you want to play.
That hasn't changed in SR4. If they did change that it would have ended up really changing the feel of the world, because that is just a side product of having a game where anyone one you meet could be dangerous to you. You're a seasoned pro with 200 karma under your belt (about year's worth of gaming)? Well a street punk with a shotgun could easily get in a lucky shot and hurt you really bad if you are stupid enough to stand out in the open in front of him.