The difference between urban decay in the US vs the UK is really all about available real estate.
In the UK, compared with in the US, we're limited in available space. What decays is quickly reclaimed. Urban decay happens in the UK to no less extent than in the US (per capita, obviously) but the signs of that decay are short-lived as urban regeneration kicks in.
In the US, the major problem (IMO) is urban sprawl. Instead of redeveloping land in the city, the trend is to spread out into suburbs and onto farming land. It's cheaper to develop than to redevelop, when you have the option to do either. It'll be centuries before the US experiences the same level of urban compression that we experience here, and until that happens urban decay will have stronger forces than urban regeneration.
Chicago, for example, suffers much less urban decay than many other US cities because of its location. It can't sprawl eastwards because Lake Michigan gets in the way, and it can't spread west because it already did - all the way to Schaumburg (and now beyond). On its south side, there's lots of urban decay all the way to Gary (.. Indiana, You Suck!) because there's very little to prevent the urbanisation of vast amounts of land. Thus Chicago looks prim until you get to its south side.
What stressed me out in the US was the Americans' failure to recognise value in historic buildings - specifically the old barns and farmhouses in rural areas. Even though in comparative terms they're often not that old (100 years or so), they're often left to decay and collapse. I began a project to catalogue the barns before they collapsed, to create a historical record at least. I was always frustrated with my American friends because they yearned to have "a history" like Britain's, but couldn't recognise their own emerging history literally in their own back yards.
