The checks and balances built into the Stack Exchange system provide a way for the community to voice it's opinion, and also to balance moderator control. There are both specific mechanisms in place and also general guidelines that allow for the community to balance out of control moderation.
Here are some things that you can do to check/balance moderator action:
Close/Reopen posts. While a moderator vote is binding on close/reopen it is reversible. That means that 5 community members can overrule a mod closure. This does not mean that moderator can't come back and close it again, but in my opinion this kind of disagreement should then merit a meta post if the moderator disagrees.
Meta. The greatest ability that the community has to combat moderator authoritarianism is through meta. Even if moderators are completely out of control and delete everything you post here (highly unlikely) or argue vehemently against you at every turn, the most important thing is the make sure there is a consistent and permanent record of your complaints and fears.
Contact the Community team. There is a small group of Stack Exchange employees devoted specifically to the overseeing and guidance of the communities that surround this and all other SE sites. This group is currently comprised of Robert Cartaino, Shog9, Grace Note, Anna Lear and Tim Post. They can be reached using the "other" option on the "contact us" page found at the bottom of every page. This form sends them an email and they both take these contacts serious and respond to them.
That said, there are some places where there is very little check on moderator power. Generally moderators take these particular things quite seriously and only do them when they feel they are in the right:
User suspensions. Because of their nature, these are confidential between the moderators, the community team and the user involved. The messages are private. However, the Community Team is copied on every message and they do read them.
Moderator Deletions. Posts deleted by moderators are not undeletable by the community. Generally moderators are interested in undeleting these if they can be fixed and flagged, but an out of control moderator could do some damage with deletions (though I believe they are both rate limited and it would trip some things internally for SE if it happened). Comment deletions are completely irreversible (for now). Though SE staff can step in if it's really important.
User Deletions. These are technically reversible, though it requires developer intervention. Moderators rarely delete users (if we do it's either by request or because they are spammers).
When a moderator is elected, or asked to serve as a pro tempore moderator they must sign the moderator agreement before they gain any moderator powers. If at any time you feel a moderator is in violation of this agreement, please contact the community team immediately. However, this agreement largely has to do with protecting your information, it does not stipulate that a moderator has the best interest of the community in mind.
High reputation users are functionally equivalent to moderators in a lot of ways and it's incumbent upon these people to be active meta participants and to balance the moderators' power.
Lastly, the best check on bad moderators is to elect good ones. Now obviously you don't know how anyone other than the current moderators are going to do, but I'm certain we will have several very good candidates in addition to the current moderators. You are correct though that there is the potential that moderators who would like to change the culture of this site may come in. However, keep some things in mind:
A significant part of this site's scope was set in day 1 by Stack Exchange inc. This is rare for an SE site, usually they are given pretty wide leeway over how to scope. However, SE came in on day 1 and said "get along or you'll get shut down." That attitude from SE hasn't changed and will not change.
Moderators are only 3-4 people in a community of hundreds. Users of this site, both high and low reputation must be strong voices on meta both in addition to and counter to the moderators of this site regardless of who they are. While moderator voices are strong and backed with some limited power to do things, they aren't the be all and end of all of how this community goes. Strong voices on meta, in chat and in comments on this site determine it's direction more significantly than the small group of moderators.
If there are site policies you feel strongly about needing to continue, make sure that they come up in the town hall discussions around elections, make sure you get a statement from the moderator in question in their nomination or the comments under it. And remember, moderators aren't (in general) acting unilaterally, they are generally acting to enforce site policy set out on meta (whether they like it or not). That means that (again) Meta is your best recourse for continuation and accountability of site policy.
While the Stack Exchange community team generally backs the moderators, they aren't afraid to say something when they aren't actin in the best interest of the community.
All that to say, there are some checks and balances, but the ultimate check is the community. Don't let the moderators you elect run you over.