Jury limitations are important because they ensure that the jury follows through on its objective: to apply the letter of the law in rendering a judgment on the accused. However, one of the problems with jury limitations is that they are very abstract and there's no way to enforce them externally. For example, the jury limitation stating that jury members must not decide based on their personal feelings is inherently unenforcable because only the individual knows the reason behind their decision. Jury limitations are self-enforced: they only work when jury members have personal integrity and agree to abide by them.
Juries have to follow the law. They cannot decide they don't like the defendant, for example, so they will vote against him. Juries have to follow jury instructions, and if they don't then that would be jury nullification.
If juries have complete discretion, then the law essentially does not exist. If juries can practice "jury nullification," for example, they can take laws made by the legislatures and declare them null and void in specific cases. This would not be fair or democratic. Therefore, there must be limits to jury discretion.
True, what are some limitations though?