Above all, I like the neat interface of the apps (mockups?) and simplicity of the cookie sensors. No charging nonsense, because they measure and buffer stuff but don't transmit. 1yr battery life. 10 day memory.
Sure, I'd love to see additional, more advanced cookies that would require charging, e.g. with built-in LED or vibration (reminding me when I enter or leave, though my phone could serve that purpose), Data, GPS (though my phone could serve this purpose, need an app that intercepts an SMS after presence is detected to auto-upload GPS data), multi-mother stuff (one at work, one at home), integration with home automation systems (someone below mentioned frequencies indicate zigbee), Zapier/IFTT support and above all some sort of data input/output API so I can import my own data points.
By the way, $222 for a mother and 4 sensors seems quite affordable to me.
The only thing that prevents me from pre-ordering a set is that the NaBazTag history doesn't exactly instill much confidence in the makers' ability to support this thing in the long run; I also wonder where the data is stored and if it's remotely future-proof (data import/export/backup).