From the Gulden Development Team:

What are our testers testing for the Gulden 2.0 update?

We have had this question come up from many community members so we thought it would be a good time to break down the parts involved. The 2.0 update involves tens of thousands of lines of code that effect every layer of the software, from the blockchain consensus, the transaction script layer to the network layer and UI. While the bulk of the code has been written and reviewed there is simply no replacement for repeated real world testing by multiple parties and this is what the testing phase is all about.

1. Phase 1 to 5 upgrade script for PoW2 and SegSig compatibility.
The above process eliminates the need for an immediate messy hard fork; allowing time for everyone to upgrade cleanly while also bringing the witnessing functinality into use as fast as possible. With the amount of services reliant on Gulden we have to keep downtime to an absolute minimum.

2. Witnessing balance in practise compared to theory.
The development team have come up with a new testnet called fastnets where each person can run their own simulations alongside our main testnet. This gives our testers the ability to fast track the results of witnessing.

3. SegSig introduces a completely new transaction format with up to 58% smaller transactions.
This new format needs to be tested in multiple ways to ensure that it works in all cases and for all transaction types and that no unexpected issues are introduced.

4. When it comes to user interface design nothing beats real user feedback.
As we reach the final part of testing our UX team and developers will be working closely with users to ensure that the interface is one that all users can use and not just the technically minded.

5. iOS and Android 2.0 compatibility testing.

6. Aside from SegSig and PoW², version 2.0 also introduces dozens of other bug fixes and various new upstream functionality.
While the main focus is on SegSig/PoW² it is important that all other functionality is also retested to not let any surprise issues through.