Looking at the new features added to the Apple Watch OS today, it seems to me that Apple was being ultra-conservative with battery life with WatchOS 1. There are lots of examples but I think the best example is the photo watch face.
The photo watch face displays the time over the top of a photo selected by the user. It seems to be a sparse watch face with one complication, similar to the Motion or Solar watch faces except with less graphics, animation and code. I cannot believe such a simple watch face was cut for time, especially considering it uses the same basic template as two other watch faces. It probably would have taken an Apple engineer an hour at most, some of which would be copying code. It's just too simple.
However, a photo takes up more power on an OLED screen than the predominantly black watch faces in WatchOS 1. My thinking, therefore, is that Apple held back anything at all which was non-essential and that could have had a impact on the battery life until they were sure that the watch battery was up to par in real world usage. Indeed, as I said, there seems to be a few of them - video, native apps, independence from the phone on known wifi networks, the time lapse watch face and so on.
Less than a day's battery life would have been a -gate level news story for the Apple Watch and probably very damaging for a new product category (let alone the first new one since Steve Jobs died). Now that it's clear that even the smaller Apple Watch seems to have at least 36 hours battery life under what its customers have collectively decided is normal use, Apple is free to start sapping the leftover battery in the service of extra features.