The Minecraft uninstaller leaves the folder tmp and the log updateLog.txt inside C:\Program Files (x86)\Minecraft (or wherever Minecraft was installed) behind.
It also leaves the registry key HKEY_CURRENT_USER\Software\Mojang\Minecraft Launcher behind, with the entries launcher_pos, upgrade_id and log_pos (if updateLog.txt exists?). Especially upgrade_id could be dangerous to keep in case someone only uninstalls programs before selling the PC.