There are tons of reasons why the original version was unplayable (fatigue system, no Auction House, no auto-attack, terrible level grind, crashing zones, glacial-speed UI, etc.) but you can find all about those in the circa-2010 reviews of the game. What's way more interesting are the many ways that the game is way more playable now and how that suggests good things for 2.0.

The game still crashes occasionally, but it's orders of magnitude more stable now. They've implemented jobs to go along with the class-based Armoury System. The UI is much cleaner and faster. Market Wards are an acceptable alternative to an Auction House. Fatigue is gone completely, in favor of "Rested Bonus". Obviously there are still problems with loot drop rates and incentivizing content, but those are gameplay tuning issues rather than client running issues. Playing the game at launch and playing the game now are two massively different experiences and I am personally enjoying myself for the most part, playing now.