If you take the Bard and remove all spell casting, they are still an OK class, but not great class. Compare them to the rogue. The fact that they are OK then have a full casting package added on as well is incrediable.
They are OP but in that way that avoids scrutiny, as they do not pull huge damage numbers and they spread out their awesomeness amongst a few areas.
I played one as an Indiana Jones, being an expert on history and avoiding traps etc. Spell casting was for help in archeology etc.
I played a Skald who was Str based, chose spells that did not need lots of Cha, but was OK at it. Wore heavy armor (feat), had two melee attacks plus Polearm master bonus attack. He felt like a fighter who was also great at lore, oh, and was a full caster.
These guys were far from optimised, but it is damn near impossible to make a bad bard.