First...GW2 is not Blizard. It's a huge leap to assume Diablo3 problems have anything to do with ArenaNet. 2 Completely different companies. And the extensive beta testing and adjustments on ArenaNet's part shows that they are serious about quality.
I played the last open beta. 60+ FPS in explorable areas. But frames dropped low with a lot of people on the screen in a big fight. Earlier poster's comment about the beta not using CPU/GPU that much is correct. Still beta, not optimized yet, essentially handling a lot of things in software. So when they optimize the final version to make full use of multicore/multi-GPU I have no doubt it will perform fine.
(* I tested on: Single ATI 6850, AMD 1090T @ 3.8, Win7 64bit, 8GB RAM)
Also, I had 0 login issues for the 2 days I played. I must have missed the problem, but I did play almost all weekend. So I don't know what people mean when they talk about GW2 beta login issues. I had none.
Based on playing GW1 I have no doubts about their ability to support millions of players on release day. ArenaNet are very focussed on releasing a quality product to their customers. Their dedication to testing and making necessary changes based on the results is not a 'technical issue'. More companies should do this prior to release. That's the way companies SHOULD be treating their products/customers.