An updated Mac mini would likely offer some improvements brought to the rest of Apple's Mac lineup over the course of the last two years, including Haswell processors, faster PCI Express-based flash storage options, and 802.11ac WiFi.
I
really hope that a mid 2014 update is true, as I'm otherwise going to have to pull the trigger on a hackintosh sometime in September (once I've finished some ongoing projects so I'll have time).
However, I really don't want to have to contend with drive issues, boot loaders etc., so if Apple releases even a modest Mac Mini update then I would jump at the chance.
Personally I expect they'll keep the same form factor for a Haswell update, with only minor changes (swap Firewire for Thunderbolt), as a case redesign would probably limit them to the 35W and 45W chips if they wanted to keep it quiet; anyone that's used a Brix Pro knows it's a pretty miserable machine thanks to the trade-offs of stuffing Iris Pro graphics into a too-small case. If the Mac Mini stays its current size then Iris Pro should be a mostly drop-in replacement, perhaps using the larger foot-print of the case for some passive cooling to avoid overburdening the fan.
I'm not sure how much I'd expect PCIe flash storage though; I guess it's possible if they want to free up the second 2.5" bay to make room for a bigger fan or something, otherwise I'm not sure how big a priority it would be for what would likely be an interim update.
I expect Apple had planned to put out a redesigned mini with Broadwell processors, but thanks to the delays they're forced to do a more modest update. Desktop Broadwell processors are likely stuck till July next year, and I wouldn't really expect the Mac Mini to be first to get them anyway, so an update now would make sense, with a Broadwell model released in a year or so.
I'm going to be annoyed if this doesn't pan out though, the wait has become torturous, as I wouldn't even been considering a hackintosh otherwise.