I think most of us would agree that the most desired feature is basic stability. But, here is a specific one, from a weewx user.
Switching preferences between "Metric" and "US" can cause records to simply disappear. For example,
If I set "preferences" to metric on the Weather Underground and look at his site (IPROVENC115), I get this:
2017-11-27 07:03:00,4.2,-1.4,1023.9,North,-9999,0.0,0.0,67,0.0,,,0.0,0.00,weewx-3.7.1,2017-11-27 06:03:00,
2017-11-27 07:08:00,4.1,-1.4,1023.9,NW,312,1.6,1.6,67,0.0,,,0.0,0.00,weewx-3.7.1,2017-11-27 06:08:00,
2017-11-27 07:18:00,4.1,-1.4,1023.9,North,-9999,0.0,0.0,67,0.0,,,0.0,0.00,weewx-3.7.1,2017-11-27 06:18:00,
2017-11-27 07:23:00,4.1,-1.3,1023.9,NW,326,1.6,1.6,68,0.0,,,0.0,0.00,weewx-3.7.1,2017-11-27 06:23:00,
However, if I set preferences to "US" and look at the same site, I get this:
2017-11-27 07:03:00,39.5,29.5,30.24,North,-9999,0.0,0.0,67,0.00,,,0.00,0.00,weewx-3.7.1,2017-11-27 06:03:00,
2017-11-27 07:08:00,39.4,29.4,30.24,NW,312,1.0,1.0,67,0.00,,,0.00,0.00,weewx-3.7.1,2017-11-27 06:08:00,
2017-11-27 07:13:00,39.4,29.7,30.24,NW,318,2.0,2.0,68,0.00,,,0.00,0.00,weewx-3.7.1,2017-11-27 06:13:00,
2017-11-27 07:18:00,39.4,29.4,30.24,North,-9999,0.0,0.0,67,0.00,,,0.00,0.00,weewx-3.7.1,2017-11-27 06:18:00,
2017-11-27 07:23:00,39.3,29.6,30.24,NW,326,1.0,1.0,68,0.00,,,0.00,0.00,weewx-3.7.1,2017-11-27 06:23:00,
A new record (07:13:00) appears.
For details, see this
weewx-user thread.
-tk
(Author of weeWX)