The RCC is very legalistic, especially when it coms to sin, Confession, Communion etc.
For example, if we commit a mortal sin, we must receive absolution before we can receive Commuinion. However, unless we do indulgences, we will have to spend some time in Purgatory.
You have some good questions here, and I'll endeavor to answer to the best of my ability. I'm sure others are more qualified, so I look forward to hearing what they have to say, as well. Firstly, though (and I know you didn't ask about this) I'd point out that there's no doctrine of Purgatory, per se, in Orthodoxy. There are the Arial Tollhouses, which are part of the particular judgment, but their literal existence is not dogma and is controversial. Whether they exist or not, I don't mean to debate, I just mean to point out that Orthodoxy and Catholicism differ. Catholicism focuses on purifying from venial sin, whereas Orthodoxy focuses on commitment to the Gospel and the grace of Christ (as your guardian angel supposedly would defend you at the tollhouses against demonic accusers). Again, please don't let this start a debate, I simply mean to indicate a difference of opinion concerning after-death experience for Orthodox and Catholics.
So here are my questions:
1. I know this is a broad question, but where do the two churches agree and differ?
IMO, it's really a difference in emphasis, more than a substantial difference, though some argue (and I tend to agree) that Catholicism emphasizes legality far too much. I think some Orthodox can overemphasize our primary view (that is, sin as an illness) as well. There's a balance to be had there, and both metaphors are biblical.
Sin is both a legal and medical reality. We are all born with a fallen nature, one that is inclined to sin (though not guilty of Adam's sin, as in the RC doctrine), and that is how it is medical. But we also struggle with choices and decide whether to sin or not...whether to breach what we know is wrong or not, and that is how it is legal. Both views are important. We should see ourselves both as "guilty" of sin and "inflicted" with sin. The focus, then, should be on treatment and rehabilitation of the sinner, rather than on some sort of punishment. Excommunication, that is a temporary ban from receiving the Eucharist, is not a punishment but a reminder of our spiritual breach with Christ because of sin. How could we physically commune with Christ when we have broken with Him in our hearts? Excommunication is used to show us the gravity of our sins, and to bring about repentance and healing...not to punish us for acting badly.
2. I noticed that some Orthodox churches only offer Confession a few times a year. If the person has committed a mortal sin, like masturbation or adultery, do they not receive Communion until they can go to Confession? What if that person dies before his Confession can be heard?
I know these are broad questions. Thanks in advance for any answers.
Traditions differ. Russian-tradition jurisdictions tend promote more frequent confession (ROCOR is usually weekly, the OCA monthly, etc.) whereas Greek/Antiochian-traditions jurisdiction are usually less so (usually quarterly). Sometimes, Orthodox will only commune once a year...usually Pascha, making an annual confession during Holy Week or right before. Others will confess/commune for the twelve Great Feasts, but not every Sunday. My point is, this differs by jurisdiction as well as by person, due to the personal and pastoral nature of the sacrament.
Mortal vs. venial sin isn't really a defined idea for Orthodoxy. We have an understanding of the gravity of different sins (evidenced by different suggested penances by differing canons), but we don't usually talk about it in these terms. Though, confession and communion are usually strongly linked for us. I know I've always been instructed that if I commit a "big" sin, to come to confession before I commune.
As far as dying...of course, the ideal is to receive confession and communion prior to death (loosely, our version of "Last Rites"). But, at the funeral for Orthodox, there is an absolution performed over the person postmortem, as well as many prayers for the remission of their sins.