The technology is only half of the story.
Companies really need to come to grips with the fact that you can't just surgically remove the beating-heart technology of an organisation, replace it with a new one and expect everything to work as normal. There needs to be buy in from the multitude of non-technical teams that need to help deliver it and most importantly the poor buggers that will need to use it.
The LSE got this really really wrong. There was outcry from all of the major trading vendors and most of the members who connected directly at the aggressive timescales and the fact that the upgrade would cost them each upwards of £1m to implement.
The initial tests were an absolute joke. Nothing worked and internal support was non-existent. Several very large vendors forced the LSE to push the delivery back and schedule more tests.
The business analysts and delivery teams within the LSE remained and carried on with the same mentality as when they were working with the Accenture-built TradElect system - that of an arse-covering bloated bureaucracy. The result of this was a massive disconnect between the developers and users and impossible timescales (it was initially touted that the new exchange be up and running in September 2010) being publicized by teams of pissed off analysts determined to ensure that Millennium took the blame when it all went tits up.
It is a testament to the paranoia within the LSE management that the Turquoise failure was initially blamed on sabotage (turned out to be a contractor screwing up a patch release). Even now, the thing is riddled with bugs and the LSE's reputation amongst it's members is shot to hell.
I hope that others take heed from this rather public failure as a result of not following the basics when running a project to replace a system that is relied upon by many.