Someone created a thread about A/Net up time with a bunch of criticism. I had a post typed up earlier today but didn't submit it and I see the thread is now closed which is a good thing. I would still like to write about GW2 up time as an MMO. So I am putting it here in the hope that none of the GW2 trolls that we have been forced to deal with lately pick up on it. This information applies to pretty much all MMO's.
As a software architect, I can tell you that while 100% up-time is possible, the cost to implement it is completely prohibitive. There are cost/benefits that need to be taken into account with any large system. High availability is governed by a number of factors from software design through to the hardware. The more reliability you want, the higher the cost and most often this cost is on the hardware side. A/Net don't have monthly subs to throw at the hardware upon hardware. Their reliability has been quite admirable so far. I normally play in the evenings and have been very impressed. To keep a system up 24/7 is extremely difficult. Everything from backups to maintenance and upgrades need to be done on the running system. That is no small feat. We have some pretty big systems and we almost always have a small amount of down time for upgrades and also experience the odd hick-up when something slips through the cracks. This even happens on one of our systems which is far bigger than GW2 from a value point of view.