Mostly what TheBlackAdder says is correct, although I like the idea that AML laws are NOT here to stay, but that is an ideological matter and he/she is probably correct for the forseeable future.
One point I would clarify:
The next AML requirement is generally that whenever a regulated financial entity (or a banker, money changer etc.) sees a transaction which he suspects might be fraudulent or an attempt to launder funds, he is required to report the transaction to the financial regulator immediately. A lot of organisations file Suspicious Activity Reports automatically for almost every transaction.
I don't believe this is true in the US. I was AML certified for institutional brokerage for many years. I never filed an SAR. I never would have, short of a colombian showing up with a suitcase full of cash or something colossally obvious. But even then I would just refuse the business to keep myself out of trouble. I believe it is clear to all that the banks, the cartels, and the various justice/law enforcement systems in place only keep drug laws in place to control the racket for themselves. This does much more harm than good for society and is a paradigm we need to collectively outgrow. None of my colleagues believed in filing SAR's either, they would just refuse business that would warrant such a thing, if it ever had arisen.
The irony, if I were laundering money, I would have filed an SAR on everyone I ever talked to, to make sure the regulators were so sick of pulling up little old ladies' financial stats that they hit the ignore button on my file. Probably the best way to obscure laundering is to deluge regulators with frivolous SARs. Maybe legit companies are doing this just as a CYA, in which case more power to them as well. It is very subjective, if a 20-something kid ran a 5-figure transaction that could be deemed suspicious activity for sure. But I don't think that is the case for most firms, they have enough mandatory regulations to bother with more, and as HSBC proved, and NYSE Chairman DeGrasso meeting with FARC out in the jungle a decade prior, the very largest firms skirt AML intentionally because it's a massive profit center.
As for MtGox and the OP, simple answer is size. MtGox at this point is running billions of USD equivalent in volume. The more money is involved, the more a regulator is concerned with potential AML issues, for obvious reasons. MtGox and BTC values are now big enough to be relevant for major transfers of wealth and thus they have to abide. The first 7-figure international wires to bank accounts would have triggered regulatory interest, for all the reasons TheBlackAdder mentioned.
Smaller firms are in theory subject to the same rules, but there is no enforcement and probably collective ignorance of the laws by their proprietors (or they are in a bank secrecy jurisdiction where there are no regulators).
The upside, it adds legitimacy to any exchange to be regulated and transparent as possible, as knowing that they ARE being scrutinized diminishes the likelihood of unscrupulous accounting, etc. In theory/perception at least. It's not necessarily a bad thing, depends on the regulators and laws applied.