This is kind of a no-brainer, but still needs to be talked about. While Google did say that Android L will be 64-bit, that doesn't mean that the Nexus 6 will be 64-bit. As we don't have many processors available right now that are 64-bit. Right now there's the Tegra K1 from NVIDIA, and a few Qualcomm processors that won't be available until next year, the Snapdragon 808 and 810. Samsung also has a few Exynos processors too, but those usually don't have LTE which wouldn't be good for the US and other LTE markets.
We're looking at the Snapdragon 801 or the 805 being in the Nexus 6, which means we may not have complete 64-bit architecture on the Nexus 6. The software will be ready but the hardware won't be, unfortunately. It would still be interesting to see Google launch a 64-bit OS on a smartphone that is not 64-bit. However, companies have done stuff like that before.
The Nexus 6 will still likely be a beast, and I don't think we really need 64-bit just yet. Not obviously we want it, but I don't think it's really needed just yet. Especially when phones are still packing in 2GB of RAM and not 4GB of RAM. 64-bit architecture doesn't really make a difference until you get to 4GB of RAM, so maybe we'll see that in the near future.