Each expansion added quite a bit of new features to the game. I don't recall exactly what was released when, but if you've only played the original then you're missing out on the hero aspects. You're able to utilize 3 heroes who are special characters you get through the campaign who you can customize to whatever class you want to supplement your own. This is on top of the basic henchmen that are also available. The reason heroes are better is that you can customize their gear for more specific abilities, you can customize their ability set based upon the abilities you've found and they can dual class just like your own hero (they all have a base class that cannot be changed). On top of that, while in a mission you can give them some basic commands for each of them, such as move here, attack this, stay here,etc.
A few of the expansions also added stronger PvE skills that focus on the enemy types specific to the campaign. Some of the skills can be "leveled" up by ranking up some sort of faction.
They also implemented a larger stash with more tabs.
Also most people end up getting the trilogy with GW:EN since the trilogy comes with guildwars, factions expansion, and nightfall expansion.