[*]Are Essentials classes easier to play?
That's a subjective answer. They do play differently. Whether that's easier or not is a matter of preference and play style.
For instance, with the uniform power progression of the original PHBs, it makes it easier to pick up any class and go right at it or have one player be able to easily answer questions for other players about their progression/powers. But with the HotF* books, the progression is different for each class so it takes more effort to try a different class or for your fellow players to know off hand what the others are/aren't capable of without doing actual research.
Having said that, in my opinion, in general the HotF* classes are 'simpler' Though no more or no less 'easier'
[*]Do Essentials classes generate less conditions than previous classes?
Not sure if they generate fewer conditions -- it depends a lot on what powers the players choose. But there is a little less condition tracking in that some of the marks and curses have been redone as auras so you'll have at least a couple less things to keep track of. Of course, if you don't have any defenders or warlocks to begin with, then you won't really see that change come in to play at your table.
[*]Do Essentials classes have less powers/abilities than previous classes?
Ah, that's a hard one to answer as a blanket response because all the classes have different progressions now.
martial classes in particular have fewer.
Some of the classes get more frequent uses of a single encounter power rather than multiple different types of encounter powers.
I will say that some of the HotF* class builds are designed better in terms of attribute requirements - such as the paladin. In the PHB, some of the paladin builds require a spread of 3 'good or better' ability scores. Compared to the HotFK paladin builds which require a spread of 2 'good or better' ability scores, allowing for more specialization.
Both essentials and nonessentials classes work fine next to each other. So if you want, just open the option of HotF* classes to the players. Some may like it, some may not, but you'll give them the option rather than requiring it one way or the other.