– HD-voice calls drop to narrow band when the network performs a handover to GSM as there is little support for HD-voice on 2G so far.
– Networks seem to be pretty cautious during voice calls and perform 3G to GSM handovers while there is still sufficient UMTS coverage. During lengthy phone calls this means that Internet connectivity is often needlesly lost, or, in the case of DTM, becomes unbearably slow for most things.
– Most network operators still use GSM A5/1 which is insecure. And even if they use better encryption, lots of organisations are using IMSI catchers and other equipment these days and can easily perform man in the middle attacks. Latest example: The government quarter in Norway.
So unless I'm really in territory which is only covered by GSM (or GSM and LTE), I really want to switch GSM off in my phone. While many 2G/3G devices still had the option to lock the device to 3G, GSM/UMTS/LTE devices usually don't have such an option anymore. Why confuse the user…!? But there's an app for that! Have a look at the "Network" app. It doesn't seem to work on all Android devices but on my Samsung S4 running CyanogenMod it works well indeed. The app is open source so I had a quick look and was surprised that it pretty much just invokes an internal configuration screen that is otherwise hidden.