First off, kudos to you for stepping up and taking over the role of guild leader from your former commander, and thanks for your question. It's surprisingly common, and one study by MMO researcher Nick Yee showed that about two-thirds of all guild leaders inherited a guild that someone else created. In a way, it's harder for a replacement guild leader to manage the guild compared to one who built the guild from the ground up. Guild leaders are like snowflakes: Each leader has his own style and philosophy, so your way of doing things is undoubtedly going to be different from the former leader's.
If you haven't already done so, it's worth it to review the guild charter and guild rules. While you really can't (and shouldn't) change the core philosophy of the guild, there might be some rules that are out of date and need to be tweaked. Including the officers and members in the process helps get everyone on the same page as well, which makes it easier on you when enforcement is needed. Doing this also gives you the chance to gently remind members of rules that they currently might not be following, which means when you do talk to them about an infraction, they can't plead ignorance.
Don't change who you are
You mentioned that it's not your style to be aggressive, and that's perfectly fine. Leaders need to be firm, but that doesn't necessarily mean you have to get in people's faces and be combative. One of the best guild leaders I ever met was also one of the most low-key. He ran one of the best guilds on the server because he was well-organized and consistent in enforcing the rules when needed. He didn't dwell on infractions, either; he responded quickly and decisively, and then moved on. Because of that, players took him seriously, and it resulted in a relatively low-drama guild that was also being one of the largest guilds on the server.
Players supported you to take on the mantle of guild leader, and they follow you because of who you are. Focus on the leadership qualities that you have rather than try to be someone you're not. You can't be the old guild leader, and the guild members might need a little time here and there to adjust to the differences in leadership, but if you're consistent and firm, they'll get on board with your way of doing things.
You are not alone
Running a guild can often feel like a very lonely existence, as if it's you against a non-stop tide of player demands and challenges to your authority. So it's crucial that you surround yourself with officers you trust. Hopefully, the officers whom the former leader chose are people that you would also select, but that's not always the case. You might need to add a few whom you trust, and you might need to talk to current officers to clarify what management tasks they handle. Few enjoy being the "bad cop" in the guild, but if you have a leadership team behind you, you don't necessarily have to be. A player who isn't following directions is going to hear not only from you but from the officers as well, and all of your voices will drive home the point that the member needs to fall in line or face consequences.
Also, form a culture that doesn't condone poor behavior. If you establish clear rules that members believe in, you'll create a guild atmosphere that won't tolerate rule breakers, so when someone doesn't follow commands, you'll have the support of the entire guild when you do take action. Going forward, it'll be easier when new members come aboard because they're more likely to adopt your philosophy and ways of doing things rather than challenge you.
You cited issues with members not following commands and not behaving appropriately in voice chat, two problems that are all-too-common in guilds. If you and the officers have talked to the players and warned them of the consequences and they're still
doing it, it's likely that they just won't change. That's when it's time to consider parting ways. Without knowing all of the details of what they're doing (or not doing), I think that if they're hindering your ability to communicate with and lead the guild during planned events, they're basically helping your opponents, and they're wasting the rest of the guild's time. You sound like a leader who's taken on this responsibility because you want to see the guild succeed and you want to see happy members, and hopefully that means the bulk of the guild is already on board with you in the driver's seat. Dealing with problem players in a firm and decisive manner isn't always easy, but by having those one or two difficult discussions, you help establish an atmosphere in which everyone is on the same page and bad behavior isn't acceptable. As a result, you won't have to work hard on enforcing discipline because it's an entrenched part of the guild's culture.
Do you have a guild problem that you just can't seem to resolve? Have a guild issue that you'd like to discuss? Every week, Karen Bryan takes on reader questions about guild management right here in The Guild Counsel column. She'll offer advice, give practical tips, and even provide a shoulder to lean on for those who are taking up the challenging task of running a guild.