The silence from them though is deafening, they won't even tell us what is wrong. As a programmer, I find the best option is to lay all the bug details out there, and allow users to understand the problem. Even when I was on here, I would explain the bug the site was facing and how I was going about it. You would be surprised what ideas are sparked off from a small comment that somebody made.
Even Reddit (which is a far larger site) will say "errr...we tried to do a backup, and the server crashed", or something like that. Not a generic message such as "Site down; we're working on it".
They have quite simply, not done enough testing and rushed the site out. Either that, or the testing they did do was flawed.
I sympathise with them a little. Rolling out a new product is a woman at the best of times when under an intense deadline.
The fact remains though, that the attention that the site brought was multiplied by City's PR machine. The PR guys wrote checks that their developers couldn't cash, as is the way with those marketing types.
Test, test, test then test some more - the golden rule of rollouts.