Summary: The EU had initially identified 29 countries where it believes UPS' purchase of TNT would reduce competition below acceptable levels (read only UPS and DHL left, FedEx and regional players too small to matter or absent altogether).
14 countries are no longer in contention, either because the EU and UPS have agreed remedies, or because the EU has reconsidered its position after further consultations with UPS.
Things are a bit more dicey for the remaining 15 countries. UPS is willing to sell all or some TNT assets in these countries, but wants to conduct the sale after the merger completes. The EU wants to know the buyer(s) before approving the merger to make sure they have the financial stability and network to provide meaningful competition, and not shrivel or fold within a few months. UPS is vehemently opposed because they read that to mean "sell to FedEx at any price."
UPS is offering to guarantee buyers access to its network at competitive prices for three years, but the EU prefers buyers with their own network because they claim such guarantees are hard to enforce and monitor.
(couldn't find an active UPS/TNT thread to add this to)