Buying a programmable thermostat to replace a manual one: any considerations?
You have to check that the thermostat is compatible with your HVAC system.  Heat pumps need the emergency heat switch, multi-stage high efficiency HVAC's have multiple fan settings, and electric baseboards may have 120/240v lines instead of the 24v wires.  For the lower end models, or for those seen in the far north or south, you may have a thermostat for only heating or cooling, but not both.

So before buying a thermostat, check the type and features on your existing HVAC system and limit your selection to the thermostats that support your system.  Then, see what features you want and how easy the system is to use.  Some systems have wifi, some have programs for all 7 days instead of 5/2 split, but if you have all 7 days, how easy is it to change the setting for every day?