In the case of Intero I made every decision for the user; there are no customization options in the mode, and it leads to long (first-time) start-ups like this. But in the end I'd rather wait a minute and have every project always work reliably (and this isn't unheard-of for IDEs). It does build a version of intero for each project environment, because the GHC version and package set may differ. This is in contrast to e.g. ghc-mod or hdevtools which is locked to a specific GHC version, so switching between projects is not smooth.
Stack has changed my development workflow, in a very positive way, and I love Intero (at least in the first 24 hours of use).
* Using my already available GHC builds, because stack's GHC builds do not work everywhere, not even across all Linux distros. If I have a musl-based Linux distro, FreeBSD, or Solaris it'll be a problem. Also, there are many users who prefer to use other builds than those downloaded via stack. This happens even if there is already the right GHC in PATH by default.
* I like cabal's new new-foo features and project file support.
* Not bailing out if I open a random .hs file where there's no associated stack config. In a sibling comment Chris says he wants it to work automagically, but then it should create a global stack file, which he says will be used if available.
* Offline mode.
* I will find more stuff I miss, once I have used stack more.