My final comparison is the Druid, the one class that I intuitively felt was somewhat weak. The Druid seems easiest to compare to the Invoker. I will compare a Wis/Con Druid who takes human-form powers with a Wis/Con Invoker.
Same statistics and armor. Both have Ritual Casting powers.
Druid has more hit points than the invoker. The Druid has a third at-will power which is a beast form power and thus allows the Druid to fight with good effect in melee, which an invoker cannot do.
Invoker has +1 fortitude, Covenant of Wrath, Armor of Wrath, and Rebuke Undead.
This is interesting. My initial impression had been that the Druid was very lacking in class features. But now that I look at this comparison, it strikes me that each the Druid class features is pretty substantial, better than any one of the Invoker class features. I'm not really sure that the Invoker class features are really better than the Druid class features.
In general, I seem to like the Invoker at-will powers a bit better (Vanguard’s Lightning > Chill Wind or Flame Seed, Sun Strike > Thorn Whip, Avenging Light > Storm Spike). The L1 encounter powers are much better for the Invoker, though the L3 powers are not. The invoker has lots of cool utility powers; the druid’s utility powers are sleepy except for Camoflage Cloak, which seems out of place. Both have solid daily powers, the Druid has one of those scary auto-hit wall spells.
So it seems to me that the Invoker is better than the Druid because it has better powers, but the difference isn't all that great. The Druid still seems like a pretty decent class, but I certainly wouldn't say it was better than a class in the PHB1.