First things first, being an officer of a guild is not so much a privilege but damn hard work. Select people to do roles who are willing to put the work in, rather than people who are self-important and want to see an "O" next to their name in the guild manager. In my opinion there are 3 major jobs, that need to be done right:
Raid Officer
Responsible for choosing what to raid to balance the conflicting goals of progression, variety, back-flagging, and gear farming. Responsible for researching strategies where they exist, and formulating strategies where they do not. Raid success hinges on this officer above any other member - a very challenging and stressful job (one small mistake can = wipe and loss of 3 hours raiding), but very rewarding when we get the big win. Our raid officer is outstanding.
Recuitment Officer
Very important role. Primarily responsible for administering the recruitment process - making sure recruits meet requirements, that sponsors do their jobs properly, and that committee members vote in a timely manner. This person should not (in my opinion) be held responsible for the number of people in the guild manager. Recruiting too quickly is just as bad as recruiting too slowly. The policy itself, and the guilds reputation are the key factors in getting the right quality of recruits at the right pace. The worst example of a recruitment officer doing their job incorrectly is spamming PoK / sending tells to unflagged people.
Loot Officer / Guild Bank
Time consuming and very important to get right. Loot going missing, or being distributed unfairly can be a major cause of drama in a guild. Work involves managing the loot policy (such as dkp tracking), distribution after raids (however it is done), keeping an up to date and historic record of all loot transactions, and detailing the contents of the guild bank. In our guild we have two officers for this, as it is a big job, one who manages dkp, and one who manages the loot and the bank. Real life accountants are perfect for this job.
This post assumes a raiding guild of course.
There are other possible positions, that can also be useful:
Public Relations Officer
Deals with conflicts with other guilds, or drama on the server boards - ideally someone very friendly, wise and even tempered.
Morale Officer
Responsible for resolving internal conflict within the guild amicably. Needs to be someone diplomatic and who doesn't take sides, but can also be firm, should be well respected by all. Even tempered.