Today I heard someone talking about creeps' aggro in the day and night, implying that it changes. I've never heard of this before. What's different about how neutrals react in the day and night? Does it change based on the creep?
They don't behave differently.
Possibly you overheard discussion about difference from dota 1 and dota 2 or someone might have had the wrong impression,until now I had feeling that creeps did change the aggro range as it was in dota 1. I really had to test it out today to convince myself otherwise.
In dota 1 at nighttime neutral creeps are "sleeping" meaning they won't attack you unless you attack them first.
As during the day neutral creeps will attack you as soon as you get in their attack range.
Only difference that makes to you is :
If you want to deny yourself with neutrals you will have to be aware if it is nighttime,it often happens that you run next to them expecting them to last hit you while they stand there just sleeping.Same thing goes for daytime and trying to run away with low health and aggroing neutrals by mistake.
Another similar thing happens when you are pulling/stacking creeps.During the day you just step inside neutrals range and they start to follow you,but during the night if you are melee hero you will have to walk all the way up to them an hit them which is 1 second more of your time that can ruin you stack/pull.
And of course for this xD
However dota 2 still hasn't implemented this mechanic,for possible reasons:
Valve thinks it brings no important gameplay changes and keeps postponing it source.
This was necessary mechanic in dota 1 due the Warcraft 3 limitations and IceFrog just decided to ditch it in dota 2(I believe this one is just a rumor since I'm pretty sure he could have changed that in W3 editor).