What's the deal with this Doctor being able to regenerate sometimes and not others?
Also I thought that the 8th Doctor was canon, so how come he could regenerate way after he'd "died" but this Doctor seems to have to do it straight away?
IIRC the time lords ability to regenerate is/was reliant in some way on the power of Gallifrey - I think they normally got about 12 regenerations on average, but can be less and can be more (an additional regeneration cycle could be given by the ruling council*).
I think the Doctor is also meant to have had additional regenerations that are not shown in the TV series (so our 8th Doctor might be the 9th, or 10th regen), and sometimes a regeneration just doesn't work without assistance (from vague memories some time lords are much better at it than others).
In short, my guess is the current Doctor isn't so great at regenerating due to the circumstances, and may be relying in part on the power in the TARDIS (which also begs the question, how is the TARDIS being powered - if Gallifray has been destroyed, so presumably has the Eye of Harmony, so are the Doc and the Tardis relying on a shared energy/bond).
I'm wondering if they are going to somehow bring Gallifray back at some point, as there are some hints that it's not "gone" as completely as the Doctor thinks (his Tardis still works**, the Daleks were not destroyed at the same time as had been suggested, and the Master survived).
*At least one of the Master story lines relates to his attempt to get more by taking over the time lord council, or stealing them.
**The ultimate joke would be if Gallifray is actually inside the TARDIS - I don't think anyone has ever mentioned how big it is inside
