The "Other" folder contains swap files and other data files such as your emails, messages and map data. The one that tends to run away and fill up the memory is map data, especially you don't delete old unused maps or if you stream your maps online. My "Other" folder has held steady for both my 920 and 820 because I only predownload the map data I require and use it offline only.
There is no need to pre-download map data just because you feel you might need it in the future because they go out of date after a few months and you will probably end up re-downloading the latest version just before you need it anyway.
Old emails also add to "Other" but not much unless you have a lot and they contain attachments.
Ok, I know what has been written and said. But anyway I don't agree! First, the memory problem of the "other" folder wasn't known. And at this stage, it is rediculous if you want to empty this folder, you have to do a hard reset. Second, not having the maps on the sd card is a mistake big time when you realize how big these maps are for the popular countries. How on earth Nokia can think to promote their Nokia (Here) maps, Drive+ etc. and safe them
in the limited phone memory of the Lumia 820, 620 etc. with the "other" folder flaw. Not really clever!!!
Apple and Samsung would have a lough 
With this in mind, someone can argue it's a Microsoft thing, but at least at the switchover from wp7.5 to wp8 Nokia should have pushed Microsoft for a change. It looks for me Nokia is not anymore the company I knew from the past.
This is not really a problem unless you intend storing plenty of files on your phone. I've been using mine half a year now and have not had any memory problems yet.
If it makes you feel any better, there isan app called Lumia Storage Check you download from betalabs.nokia.com which allows you to see exactly what your memory is doing. It also gives you the option of saving map data to the memory card if you choose to do so.
No, you don't need the phone unlocked or rooted. The storage check app is available to all free of charge from betalabs.nokia.com. There is an option within the app to save map data to memory card if you wish to do so.
I am a strong believer in non operator branded phones. Almost all my phones are non operator branded and I don't experience most of the issues I see here from branded handsets. The problem in my opinion stems from the firmware localisation carried by be the networks when branding. I have had no major issues with both my 920 and 820 and I have had them since launch last year.
I know branded handsets are cheaper to buy because they are usually subsidised but the subsidy is no more than a hire purchase agreement where the customer ends up paying for the phone anyway over the duration of the contract. IMO, better to buy your own and get a sim only tariff from your provider.
