I was amazed it came back at all actually, last season was a complete train wreck with all the plot holes involved around explaining the black out, and then the infuriating rinse and repeat of stacking the good guys up against an army and they always seemed to come out ok. This season, while not perfect by a long stretch, I felt they at least shored up most of the glaring issues of the first, and was much more watchable. In fact, I grew to like most of the characters this time around, save Tom Neville. Dude is a great actor, but his story line was pretty much the embodiment of what was wrong with last season.
With that said, I really felt the show had finally hit its groove, and I had become a fan. They had enough complexity with all the different nations and factions that really this could have been a show that went on for a few seasons with no need for any filler. I would totally understand their cancellation if they were actually making room for stuff that was better, but looking at their list of shows coming next fall that are new, nothing really jumps out at me that would be any more successful.