There's people out there with more insight on this than I have (word up The Big Lychee blog), but I'm struck by a singular thought: this decision will literally leave the PRC government in the position of having to explain why it is that a pro-democrat like Albert Ho could run in the 2012 election, where he stood no chance of winning, but in an electoral system where he could win, the new system will almost certainly block him from running as "unpatriotic" (i.e., not a supporter of the Chinese Communist Party). No doubt the PRC government will be no more phased by this contradiction than they are by the myriad other contradictions of modern-day China, not least of which is the anachronism of their rule, but people in Hong Kong will not so easily dismiss it.
Journalist Mark Mackinnon thinks this violates the spirit of the handover agreement, but personally I think that already happened when the CCP issued a white paper saying that they could end Hong Kong's autonomy if Hong Kong did not "fully respect" the mainland's political system. The promise of 50 years of unchanging autonomy is empty if the government says it can change it at any time it likes on such vaguely-defined grounds as a lack of "respect".
EDIT: This also seems pretty relevant -
for those who say #hk dems brought this on themselves by demanding too much, being too uncompromising, I say look at Macau
— Yuen Chan (@xinwenxiaojie) August 31, 2014
[Picture: former Hong Kong chief executive candidate Albert Ho addresses supporters of Occupy Central, 19 June, 2014, via Wikicommons]