(1) This is a victory for terrorists, but not necessarily for terrorism. Or to put it another way, whichever side won, it would have been at least a partial victory for terrorism. First of all, Hamas has more or less observed to a cease-fire with Israel for the last six months. It's resounding victory can be seen in part to as an endorsement of that policy. Second, as I noted previously, the most popular Fatah figure, Marwan Barghouti, is himself a terrorist, as were other Fatah candidates. In fact, Debka published a campaign photo allegedly circulated by Fatah showing Barghouti in a jailhouse embrace with a terrorist convicted of murdering schoolchildren in Israel in 1970s. Not to mention that the lethal Al Asqa Brigades are still affiliated with Fatah. A Palestinian voter wishing to vote against terrorism really didn't have much to and choose from between Fatah and Hamas. But Palestinian polls showed that 54% of Hamas voters want to reach a peace agreement with Israel.
(2) If I were voting in the Palestinian elections, I would have been sorely tempted to vote for Hamas, even if I rejected their Islamicist views and their policies toward Israel. This is because the Palestinian Authority is one of the most corrupt and incompetent governments in the world. The PA has received billions of dollars in foreign aid but one would be hard-pressed to find a single school, hospital, park, or other public building built by the PA with that money. The money has disappeared either into the hands of corrupt officials, or into paying salaries to the tens of thousands of Palestinians who work for the government, who do basically nothing but are thus bought off by Fatah. Indeed, the election results almost certainly overestimate the PA's real support because assumedly government patronage employees and their dependents voted for Fatah. Because of the PA's low standing among the Palestinian public, it probably was not capable of reaching any kind of agreement with Israel, much less disarming Hamas and Jihad, and even if it were capable of reaching such agreement, it would not have had any legitimacy among the public.
(3) On the other hand, if Hamas decides to negotiate with Israel, any agreement it reaches will have legitimacy among the Palestinian public. Why might Hamas change its views toward recognizing Israel and cease engaging in terrorism? Now it is responsible for the welfare of the Palestinian public. The Israeli government has not used even a fraction of its retaliatory capability against the Palestinians. If Hamas encourages or even tolerates terrorism, Israel can start by refusing to transfer taxes paid to Palestinians by Israeli companies to the PA. Israel can also shut off the electric grid, the water supply, cell phone service, close the borders to Palestinian goods heading to Israel and Europe, etc. Hezbollah now rarely attacks Israel because it knows that Israel will retaliate by bombing power stations in Beirut, and the Lebanese public will blame it for their suffering. Similarly, if every time a Qassam missile lands in Israel, the electricity goes off in Gaza for a week, it might not be very long before no more Qassams land in Israel. Israel couldn't engage in such tactics with the PA because the government had too much international legitimacy, however undeserved, and because Israel still has some hope that not being in an official state of war with the PA, and having the relatively moderate Abbas as president, a deal could still be worked out. But if Hamas doesn't change its stripes, Israel won't hesitate to wreak havoc on the Palestinian areas. Hamas will not be able to play the double game that Arafat was able to play of negotiating talking peace while keeping terrorist groups on a long leash; nor will it be able to get Europe and the US to restrain Israeli military action, as Abbas has, on the premise that it just need a little more time to organize itself against terrorism.
(4) Related to the above posts, the Palestinian authority is broke, and without the transfer of Israeli tax money and European and US foreign aid, it's not all clear how Hamas is going to pay thousands of armed Fatah men on the PA payroll, much less pay its own civil servants and otherwise run the government. The only way to get this money will be to renounce terrorism and recognize Israel. Hamas, I believe, would have much preferred to have received the 45% of vote that was predicted. That would have given them the ability to block the PA from disarming them, would have ensured they would have received several ministerial portfolios, but still not left them responsible ultimately for the welfare of the Palestinian public.
(5) If Hamas turns out to be unwilling to turn itself into a non-terrorist movement that Israel can reasonably deal with, Israel will have no choice but to absolutely destroy the Palestinian government. If that's what happens, the international community may need to rethink the whole idea of a sovereign Palestinian state. Professor Inbar of Bar Ilan University has suggested that Gaza should be confederated with Egypt, and the West Bank with Jordan, with these governments, which have peace treaties with Israel, having security responsibility. In my opinion, such a plan would make Palestinian independence/autonomy much more viable from both a political and economic perspective. It would obviously take significant change international attitudes to move toward that solution, but an unrepentant Hamas terrorist government in the Palestinian areas might just move opinion in that direction.
UPDATE: Further evidence that the Hamas victory is not necessarily a disaster. Israeli stocks, as reflected in this quote on the closed-end First Israel fund, are calm.