We don't honestly know 100% of what needs to be completed pre-2.0 in order to see it, but here are some suggestions of content we *think* will either be gone/or change between now and 2.0.
1. You need to get your Goobue Mount (you can get by talking to an NPC once you hit level 30). Before patch 1.23 is introduced.
2. Main storyline. This is an assumption on my part, but the main storyline seems to work well with this whole 'Dalamund Thing' so if you are leveling a class to 50, might as well do this anyway.
3. All 'purple' achievement quests/ 'purple' aytherite quests. These are random in game events that usually only take place during 1 patch. They typically involve talking to an npc who appears with a mysterious purple aytherite and then teleing via the aytherite to a garlean battle. The 'purple' achievements can also be earned from killing Garleans in the random attacks they are doing outside each of the towns.
4. Guild-company missions. Again, this storyline is very dependant on the 'fall of Dalamund' storyline.
5. In-game seasonal events and their achievements/items are only obtainable during the event (Summer Event is the only one left between now and 2.0)
6. Some think primal battles might also be on the 'until 2.0' list. However, as they are part of the relic questline (or at least Ifrit and Garuda are) I am unsure about this. Although as they are fun battles to complete, might as well knock them out when you have the time.
This is all I can think of for the time being. Really, SE hasn't given a definative list.
Edited, Jun 25th 2012 1:13am by Kinseykinz