The reason I've seen most agile projects get into trouble
I've seen agile implemented with varying degrees of success, and produce products with varying quality, quantity and delivery time (not necessarily exclusive). It's been my observation that the main thing that causes agile to be majorly neutered is when people start seeing it as an "answer" that will make their life easier.
One particularly disastrous project I saw (and was involved in cleaning up) the team said "We're going to be agile now, so we don't need to plan anything". They then proceeded to each take a bit of the system, not talk to each other, and code it. In that case they didn't know what agile was, but even when people do, they want agile to just go off and fix that problem for them. I'm sure everyone on an agile project has encountered someone saying "We're agile now, so I don't have to do x". As the article said, that's not really what it was about.
In my opinion the only way agile works is if everyone in the process, from analysts to designers to developers to testers to managers to customers, invests time and effort in doing their bit for the process. Agile requires more ongoing effort than waterfall because with waterfall you produce the deliverable for your step, then head off down the pub. Agile requires everyone at every level spending time every week from day 1 to quite a way after day of release.