One hundred gold is a lot of money. Give that amount to anyone below level twenty and they will love you forever. Or at least until they burn it all on enchantments and a mount later on. Even to a level eighty, it's still a lot of money. We have to train up professions, deal with repair bills and a multitude of other things. Why on Earth, then, should a guild master waste one hundred gold (or however much your respective game charges - but I'm going to focus on WoW) by creating a guild bank?
Now you may be thinking 'Hang on! Are you saying that we shouldn't have guild banks!?'. Reassuringly, I'm not suggesting anything of the sort. I actively encourage any organisation, regardless of size, to get a bank. Now Blizzard have finally implemented a way in which you can safely monitor the ongoings of such a useful feature, there is absolutely no reason why you can't take advantage of it. A good guild should be utilising a bank from the get-go, because if you're not, its a waste of a hundred gold. I insist that everyone should have access to at least one tab. The moment they join, they should be able to withdraw money and gold (even if it's only a stack of items and one gold per day).
You should tie in ranks with banking, too. The main monetary motivation behind people pursuing a promotion will be the ability to access more of the bank. Higher ranks should have greater freedom over the bank, from the amount of gold they can take out, to the tabs they can see. By giving people the right to withdraw from the moment they get into the guild will only make them want more and more. How do they get more? By contributing to the guild and pleasing you in order to rank up, of course! You are giving people an instant return for their own investment in the guild and loyalty builds as a result. Therefore, when it comes to them deciding if they want to stay in the guild or not, the bank (and how well it's handled) will weigh heavily in such an analysis.
However, if everyone has access to the bank from Day One, you're going to run into some security issues. These can be easily avoided if you follow these tips:
Keep an eye on the logs.
There are two logs which come with a guild bank. One tracks items whilst the other handles money. Anything that goes in or out will be detailed and it is your job to keep an eye on that information. By seeing what gets withdrawn, who withdraws it and at what time, you can find out a lot about your guild. Is someone using up their entire gold limit each day? Is someone else putting in a lot of gold without taking any out, and therefore actively demonstrating their allegiance to the guild? People should be sanctioned or rewarded for their guild bank activity as they would if they behaved well or badly during an event. Furthermore, checking up on the flow of money and items in the guild assures that their security is not compromised.
Don't be afraid to approach people if you see any abnormal activity.
Obviously if someone is blatantly stealing from the guild at any given opportunity, then kick them. However, such extreme measures can be avoided just by talking to problematic people. Send them a whisper (or mail if they're not online) stating kindly that you're not a charity, that they are expected to put back just as much as they take out and politely inquire as to what they're actually doing with all that money and/or items. A lot of people will simply not reply, but cease their negative actions outright. If they do not change their ways, kick them. The other side of this is if you see someone putting in a lot of useful goods, or boosting the gold coffers. Don't be afraid to congratulate an individual who is willing to sacrifice their own fortune for the good of the guild.
Keep the really valuable stuff locked away.
Anything that is of any real worth to the guild should be safely stored in the higher security tabs. Never give anyone other than yourself full access to the bank and only your most trusted members should have the ability to view and withdraw from all tabs. This works in the best interest of security and gives your lower ranking members something to work towards. No one can be totally trusted and I'm sure there's many testimonies out there about how a naïve guild master gave full banking privileges to a subordinate, only for them to totally empty the bank and transfer away from the server. Trust members, but always exercise a level of caution.
As for what exactly you should< keep in your guild bank; that's up to you. Since everyone appears to be respeccing their professions at a moment's notice nowadays, reagents for all purposes should be freely available. Don't worry about filling up your tabs the second you buy them, though. Remember that its a guild bank, and given the fact you've probably spent lots of money already on self-improvement to lead the guild efficiently, let your guildies fill up the storage with what they want. Obviously if your space becomes full of grey and useless items, then take action. Otherwise just let people do what they wish. Give people the freedom and the option to donate, and they will.
Only open a bank if you're willing to let everyone use it.
Everyone should have access from the moment they join, albeit a very small amount.
The ability to make use of more parts of the bank is a huge incentive for people to fight over a promotion. This leads them acting in a way that is beneficial to you and the guild.
Check your logs daily, in order to see who is withdrawing what and to make sure security is not compromised.
Communicate. A guildie who donates gold should be rewarded, whereas one which takes without giving should be punished. Talk it over with troublesome individuals and try to find a solution.