Is the cup half full, or is it half empty?
So far, the European Union and Turkey have agreed on several key steps toward a landmark deal to end Europe's migrant crisis. They agree that the EU can send back irregular migrants if it accepts one Syrian from Turkish refugee camps for each Syrian returned. And they agree on discussing more money for Ankara, faster talks on joining the EU, and visa-free travel for Turkish citizens.
But here are four things they have not yet agreed upon, any one of which could derail the delicate negotiations as EU leaders and Turkish Prime Minister Ahmet Davatoglu meet in Brussels on March 18.
The Cyprus Question
Turkey has sought to join the EU for decades, seeing it as an engine for economic growth. But Ankara's progress has been slow thanks in large part to its tensions with EU-member Cyprus.
Those tensions date back to 1974 when Turkish troops occupied part of the island amid violence between its Greek and Turkish-speaking communities. The resulting partition left an internationally recognized Cypriot rump state in the south and a breakaway northern entity recognized only by Turkey.
Since joining the EU in 2004, Cyprus has vetoed talks between Brussels and Ankara about several key issues that are part of Turkey's EU bid. Nicosia cites Ankara's refusal to recognize it as the island's sole legitimate government and to give it the same access to Turkish ports and airports that other EU states enjoy.
European Council President Donald Tusk has acknowledged Cyprus's power to now veto any migrant deal. He said on March 17 that "the agreement must be acceptable to all 28 member states, no matter big or small" -- a clear reference to the Mediterranean island.
What Turkey is willing to offer to persuade Nicosia to not exercise its veto power is now a central question in the talks over the migrant crisis.
So far, Turkey has taken a hard line. A senior Turkish official told media in Brussels on March 17 that "countries like Cyprus should not be allowed to block progress."
Human Rights Concerns
If the Cyprus question has long bedeviled EU-Turkish relations, so have human rights concerns. They too pose problems for a migrant deal.
Under the European Convention on Human Rights, refugees who are refused asylum in Europe can only be returned to a country that is safe and which can guarantee their basic rights of access to health care, education, and work.
Some UN officials have questioned whether Turkey is a "safe third country" for refused asylum seekers. One reason is that Turkey does not offer Syrians asylum but only a lesser form of international protections.
At the same time, human rights activists say that Turkey does not provide work, education, and health care for all the refugees who are there.
For just such reasons, the EU's plan to return irregular migrants to Turkey has been met with concern by some member states.
Lithuania's President Dalia Grybauskaite said this week that the package was "very much on the edge of international law."German Chancellor Angela Merkel has said that Turkey has to meet international standards of protection for all migrants for the deal to work.
Yet another reason the EU has been slow to negotiate membership for Turkey is concern it could lead to Turkish workers flooding westward.
That issue is back as Turkey now demands that its 70 million citizens receive the right to travel -- though not work -- in the visa-free in the Schengen zone by the end of June.
As part of any deal, EU officials insist that Turkey must assure its citizens have biometric passports. They also have linked visa-free travel to Ankara agreeing to 72 conditions, including changing its own visa policies to make it harder for migrants to enter Turkey from other countries.
But even that may not be enough to reassure some EU states, such as France, which have balked at the visa-free travel idea partly out of fear of a backlash from anti-immigrant parties at home.
One-In, One-Out, But How Many?
A key principle in the migrant crisis negotiations is the idea that, for each Syrian who travels irregularly to Greece and is returned, the EU will accept another Syrian directly from refugee camps in Turkey.
But what remains to be worked out in this "one-in, one-out" formula, is how many legal Syrian asylum seekers the EU would ultimately accept.
The latest EU draft for a deal between the European Union and Turkey speaks of accepting 72,000 Syrians from Turkish refugee camps. But whether Turkey will consider that sufficient when it is struggling to host 2.7 million registered refugees from Syria, plus an unknown number of unregistered refugees, is unknown.
Key to answering the question will be how Ankara views the EU's plans to speed up the disbursement of 3 billion euros ($3.4 billion) already promised to help take care of Syrian refugees in Turkey.
The EU draft says that, if this money is spent "appropriately," the EU stands ready to provide up to 3 billion more euros for Syrian refugees. But whether Turkey will accept such a conditional promise to provide more aid is uncertain.