That's incredibly inflexible, which Diablo 2 seems to realise, hence why you can map all of your abilities to F1-F8. See, abilities are mapped to the mouse buttons, and you can only have two active at a time. This brings us to how you actually use your abilities, back before Blizzard had refined its UI and hotbars. Ultimately, it's just more fun to fiddle around with, and the differences between builds are more dramatic, more meaningful.
Every time you unlock a new ability or rune, you've got something that can fundamentally change how your character plays. See, while there are fewer abilities, all of them can be augmented with runes, dramatically changing them. This flexibility is borrowed from Diablo 3, however, which also makes me crave the other ways in which Diablo 3 improves experimenting with builds. They might not all be viable for the endgame or PvP, but that is less of an issue now that you can easily reroll your character. You level up fast, and there are so many different ways to make whatever class you pick your own. I should add that I don't think Diablo 2's character progression is bad. If you've been overcome with nostalgia and decided to chase the Wanderer again, take a look at our Diablo 2: Resurrected class guide to help you pick between the Amazon, Assassin, Barbarian, Druid, Necromancer, Paladin or Sorceress.
