What rimsy said, to get 4 hours on screen time with a constant wifi or/and data connection over 2 days with gmail etc. and auto brightness is pretty good going.
A wifi connection is fine and shouldn't drain power too much (I used to get a 2% battery loss overnight - a little lower now given the 2 year old battery, but still 4-5%). Poor reception can drain a battery quite quickly, and if you're using a data connection only, especially a poor one, it will just rip right through it. Still, despite the limited size battery in the N5, I can't say I've ever been found wanting or even the need to buy a portable charger.
It would be but when you've got phones like the Xperia Z3 pushing 7 hours we should expect that from all phones.
We shouldn't expect it, but we all want it. But few phone manufacturers sell phones based on battery size, they sell it on form factor, and think slimmer is better (I disagree of course, but I don't work for a phone manufacturer). How many S2/S3 extended batteries do you think Samsung sold? I bet it was less than 5% of the phones sold. Hardly an ideal gauge, but still, if it mattered to most people, most people would do something about it.
Motorola made the Droid Turbo with a huge battery, but it's only been sold by Verizon in America. Big phones do happen, but they clearly feel at the moment that the market is so limited that it's not worth it rolling them out world-wide.
Sony's approach was the best of both worlds. They had a large phone (partly due to the speakers I assume) so they were able to fit a large battery while keeping it slim, but I doubt they designed it to fit a large battery first and foremost and then designed the rest of the phone around it.
For us techies who use our phones regularly most of us will consider the battery important, and something we always look for when a new phone is released. For the general public it seems less of a concern, or certainly the manufacturers think so.