Just to clarify, the mood level always begins at 'people are pleased with you' (though the designer may have changed this by playing the game before saving). The difficulty settings come into play when mood is recalculated, the first opportunity for which is mid-January, provided that there is a population by then (and the designer hasn't used up the time before saving).

The numbers translate as follows:
Very Easy - people love you
Easy - people are extremely pleased with you
Normal - people are very pleased with you
Hard - people are pleased with you
Very Hard - people are indifferent to you

If you do nothing to maintain sentiment these levels will start to fall very soon, and we all know what that means at Very Hard. The principle effect on a fledgling population is the maximum immigration rate, which are:

Extremely pleased or better: 12 per 'cycle' (192 per month)
Very pleased: 9 per 'cycle' (144 per month)
Pleased/indifferent: 6 per cycle (96 per month)
Annoyed: zilch.

In a city over a certain size, and tests bear out goonsquad's figure of 400 population, 'pleased' gets 144 a month instead of 96 and 'very pleased' gets 192 instead of 144. The other levels remain the same.

So whatever difficulty you play you cannot get more than 48 in the first two weeks (and then only if you are very quick). Note - these rates apply to 'new arrivals' reported by the Population Advisor, not immigrants actually moving in, and are subject to adequate vacancy (and no war)