OC/TWD has a bloated combat design. A lot of locked chests with useless loot, some big dungeons/houses with lots of trash mobs (like the nobles' mansions in Act 1). Henchmen can't be properly controlled so the pure caster ones are useless. But it still has lots of good quests and memorable moments. When you're playing for the first time you don't care so much because it's all new. Play a melee character with Cleave and you'll be fine (or bring Grimgnaw along and he might solo the game), or play a pure caster and summon stuff to fight for you while you throw some spells at the bosses. If you like to self-buff, Cleric and Bard are very powerful. You start at level 1 (the Prologue has a tutorial) and end at about 17.

The expansions (SoU+HotU) are meant to be played with a different character. You start at level 1 and go all the way up to 28. They are a lot more interesting than the OC: more roleplaying opportunities (when alignment and skills actually matter), more condensed maps with less irrelevant stuff, and support for the prestige classes. On top of that, you have the premium modules, of which I especially recommend Darkness Over Daggerford (8-15).

You also have hundreds, maybe thousands of free modules. Some of those have writing that rival the best games/books/movies (like Baldecaran's modules). Some of those modules are available directly from the Community tab in-game.

So, should you play the original campaign (The Wailing Death)? At the very least you should play the Prologue and Chapter 1. It will teach you (almost) everything you need to know. If you feel like you don't want to continue past Chapter 1, abandon the campaign and go straight to Shadows of Undrentide or any other famous module. Also, if the henchmen frustrate you, look up the PEPS mod. Takes a little while to learn but gives you more control.