What are the disadvantages of jailbreaking iPhone 4 even if it's legal?

What are the disadvantages of jailbreaking the iPhone 4 even if it's legal? I'm interested in disadvantages in any and all terms, but especially about the warranty, future updates of iOS, and bricking?

Will I have to jailbreak after each iOS update?

Disadvantages:

• Potential reduced stability of the iDevice
  • More-than-usual application crashes
  • Unusual application behavior
  • Buggy connections (like WiFi), especially on beta releases of jailbreaks. (In other words: wait until the people who write the jailbreak say it is ready for regular use. Otherwise, expect occasional pain.)
• Potential failure of application features
  • Most notably of late was the issue that iBooks would lock users out of DRM'd material if they were jailbroken. It could determine this by attempting to run unsigned code, which if it ran, would let iBooks know the device was jailbroken. A hack was released later to resolve the issue, but there is always the chance another application will also implement such features.
• Inability to receive Apple support
  • Apple doesn't want to support any jailbroken device (nor should they be required to do so). If you run into something critical, restore/upgrade to an unjailbroken state first, at which point Apple should be able to help. (Although chances are very good the particular problem will also have gone away.)
• Increased Memory Load
  • On the iPhone 4 it isn't as obvious, but it is (IMO) very obvious on the iPad 1 -- when you add services that must always be running (e.g., SBSettings, etc.) you increase the memory load on the device. Apps that were previously very close to running out of memory may do so when they least expect it, leading to unexpected crashes. This isn't particularly the fault of the jailbreak itself -- but typically one doesn't jailbreak and then not install other jailbreak-specific apps.
• Increased Likelihood of Strange Behaviour after iOS upgrades
  • Example: I had a 5-row keyboard installed on my iPhone 4. After I upgraded to 4.3.2, I ran into an immediate problem: whenever I needed my keyboard, most of it didn't appear, and most of it didn't work. Apparently parts of the 5-row keyboard configuration had stayed around after the upgrade, and the only fix was to go into General -> Keyboards and switch to the English keyboard. (At the time there was no untethered jailbreak for 4.3.2, or I would have continued the jailbreak, which likely would also have restored keyboard functionality.)
• Increased Security Concerns
  • Jailbreaking can oft mean adding software that hasn't been through Apple's vetting process, which may make it easier to get malware or other software that does "bad things".
  • If you add SSH functionality, be sure to change the root and mobile passwords; otherwise you'll get hacked in no time flat.
  • On the other hand, you can also sometimes get increased security. IIRC, there was a Cydia app that was used to add security to the device (essentially something Apple should have done, but hadn't to that point).

Personally, the pros of jailbreaking far outweigh the possible cons. I've had far more strange behavior than typical for an iDevice, yes, but I also expect it and know how to deal with it. Some features are such that I really expect that Apple should have implemented them to begin with, and sometimes Apple does occasionally remove one or two particular jailbreaking reasons. But until they satisfy my (very) long list of jailbreaking features, I'll keep on doing it.

Normally you have to jailbreak after each update. Apple is willing to close the open doors.

Disadvantages: Normally your iPhone is safe. With a jailbreak it COULD get a security leak, by granting root access with a default password. Without changing that password, someone can get root access with this default password.

As with what @New said, your phone will have to be re-jailbroken after every software update. This also means that you have to either be willing to give up your jailbroken applications until a jailbreak comes out for each new version, or wait to upgrade to the new version until there is a jailbreak available.

Another (in my opinion, very annoying) issue with jailbreaking is that your jailbroken applications and settings will be lost after every update. Even if there's a jailbreak available for the version you're installing, your phone is initially updated by iTunes to the normal non-jailbroken software, then you jailbreak it again. In the meantime, all of your applications that you've installed through means other than the App Store have been removed. There are products like PkgBackup to back up your jailbroken applications and settings, but the restoration is far from perfect; scanning through the list of hundreds of .plist files to figure out which ones you want to restore is not a simple, quick, or obvious task.

I do jailbreak my phone, but I don't particularly care for having to live with these issues.