It's easy to recruit people here. You fly them out (at any time of year, even winter) and the weather is typically sunny and pleasant. (yes, in the winter, it's usually sunny, dry, and mild temps)
Good luck doing that in Detroit.
1) Land is available, but labor and building companies are still not fully recovered from the 2008 bust, and this has created a sharp supply issue with new housing to accommodate the hordes of newcomers.
2) This issue is quickly on the way to being remedied, because the housing prices are high enough to produce huge profits for builders, and the national companies are quickly getting their infrastructure in place.
3) If anything, housing prices are going to retreat downward, (probably not a crash due to demand) and Amazon would help cement the decision for big building companies to devote more assets to correcting the supply issue.
That said, I wouldn't mind seeing Amazon HQ2 locate here. While I doubt I would personally want to work for Amazon due to its reputation as being a lousy place to work, it would probably boost the tech scene and salaries. I think Denver's cost of living is going to continue its steep trajectory with or without Amazon.