The big debate about the Fatah-Hamas unity agreement of earlier this month is whether it will increase the chances of Israeli-Palestinian peace, or weaken them.
On the former, it is argued that a unified Palestinian leadership is a sine qua non for a peace agreement. Israel couldn't make peace anyway with only half the Palestinian Authority (PA) and half the Palestinian population that includes only half the Palestinian territory. Plus, bringing Hamas into an arrangement with Fatah will moderate it: Hamas sees what is happening to autocratic rulers all around the region, and tying itself to a faction that promotes good governance and negotiations over fundamentalism and commitment to violence can prolong its own life.
On the latter, it is asserted that Hamas will be the one to swallow up Fatah, so that its radical Islamist agenda will overwhelm Fatah's secular priorities and Salam Fayyad's focus on building foundational economic and political structures.
Putting aside the question of whether the unity pact is a good thing or not, I'd argue that it's hard to see how it can work in practice. It looks good on paper to have the two Palestinian factions agree to work together, but how, exactly, they will do so is unclear for several reasons.
First, Hamas hasn't given any strong indication that it is now willing to recognize Israel, negotiate with it, and renounce violence. PA President Mahmoud Abbas's office has indicated that Hamas doesn't have to do any of those things for the agreement to work. But no Israeli government -- Likud-led or otherwise -- has the will or the political support to negotiate with Hamas unless it does so.
Second, no Palestinian-Israeli agreement will endure without the support of the international community, and particularly the United States. But the U.S. cannot deal with Hamas in its current form: there are legal and political constraints on it, and already Congress has warned that if Fatah shares power with Hamas, American aid to the PA will be endangered.
Third, the details of implementation of the unity deal are vague. It is supposed to put in place a government of technocrats to overseas plans for elections next year. It's hard to see the two sides, though, not fighting over the selection of these officials. It's also hard to see that either side would accept the results of the elections if they are the losers from it -- indeed, we can expect fierce competition (or fighting) between them once the campaign gets underway.
Fourth, at least part of the reason for the current stability in Palestinian-Israeli relations is based on good security cooperation between Israeli and West Bank security forces. I cannot imagine that Israel will work with Hamas forces, and rely on them to help maintain the prevention of terrorism against Israel. Not to mention the unwillingness of Israel to share ideas about security with Hamas -- from Israel's perspective, it would be like the hens explaining to the fox the mechanisms of the locks on the henhouse doors and windows.
Fifth, Hamas itself is riven with internal differences that complicate both Hamas-Fatah and Hamas-Israel relations. Hardliners are absolutely committed to terrorism against Israel; they firmly believe in many of the anti-Semitic elements of the Hamas charter; and they seem to consistently and publicly proclaim their commitment to an eventual "liberation" of all of Palestine, not just the West Bank and Gaza. There's no indication yet that the moderates within Hamas can overcome these ideological constraints, assuming they want to.
One might be able to list several other reasons. The point, though, is that despite the private motivations pushing Hamas and Fatah, it does not appear as though regional conditions are conducive to its success right now, and therefore we should temper our optimism. As we know from past experiences, such conditions can easily derail progress on peace talks unless and until all the players have the willpower to overcome their adversaries and rivals and see it through to the end. It unfortunately doesn't appear that the current leaders of Israel, Hamas, and Fatah have that strength of will and self-discipline.