Warpriest is a cleric build. It does have at-will powers (they're listed with the domains - Sun or Storm in Heroes of the Fallen Lands) as well as encounter, daily and utility powers. It's what's sometimes referred to as an AEDU built (at-will / encounter / daily / utility powers) - the traditional 4e character build.
Essentials presents new options. All of the previous classes are still fine and pretty much unchanged (there have been constant rules updates since the beginning of 4e). Some of these builds feel a lot like the older builds (Warpriest, Mage). Some of them are radically different from the older builds (Knight, Slayer, Thief, Hunter, Scout).
The radically different builds abandon the AEDU structure. Instead of having at-will attack powers, they're built around melee basic attacks or ranged basic attacks. They have other at-will abilities (either moves or stances or tweaks to basic attacks). They have a single encounter power and get more uses of it as they level up. They don't get daily attack powers.
These new builds are simpler to construct and to play than the AEDU builds (opinions vary; that's mine). People who want the complexity of AEDU classes probably won't like the martial Essentials classes, but the original classes are still right there. Some people freak out about Essentials being a half-edition; it's not, in my opinion.
As for monsters, you'll find that earlier monsters just don't deal enough damage (there are a few other issues, too, but that's the big one). When I'm running adventures that were published with older monsters, I update the damage they deal (and I add some extra abilities to solo monsters so they don't get locked out of a fight so easily by action denial effects). Page 40 of
this document will give you the updated monster stats to use.