Why are there only five classes? Considering that ADOM has 22 classes, that's a net loss of 17 classes. Quite significant, if taken at face value.
Second, and more importantly, classes in Ultimate ADOM are not like classes in ADOM. Let's have a look at Alynna the elven archer up there. They can Sneak, Butcher things and Cast spells among other standard actions from the very beginning - because they decided to spend their skill points in these trees:
The Archer class will receive extra skill points to be distributed to missile weapons. Right now we have a selection of Bows (Alynna's choice), Crossbows and Slings in the game, but there are plans for including throwing weapons, too. There are advantages of specializing in a single category of missile weapons, as this makes unlocking powerful skills and boosts more easy. A different archer might decide to spread evenly between crossbows and slings, though, remaining versatile and ready to deal with monsters that are more resistant to being peppered with pointy sticks.
So, yes. Right now, there's only five classes - but no Wizard, no Fighter, no Adventurer, no Priest and no Archer needs ever be the same.
We can't wait to read about your favorite combinations and the adventures they will encounter!