The Reddit front page is hopeless. It's like the Firehose on this site. The Subreddits are where the action is really at, and most seem to have enough moderators who are interested in the specific subject matter to handle the large number of dupes, bin spam, etc. that get posted. Here are some examples of great parts of Reddit:
http://www.reddit.com/r/fffffffuuuuuuuuuuuu/ popularized Rage Comics. Anyone can make a comic and the moderation system works pretty well for those.
http://www.reddit.com/r/todayilearned/ provides interesting trivia facts
http://www.reddit.com/r/science/ has far more relevant and interesting content than the Science section here. Same with the http://www.reddit.com/r/gaming/ section.
http://www.reddit.com/r/LetsNotMeet/ was created to let users share true stories of creepy things happening that are just too personal or random to share elsewhere.
http://www.reddit.com/r/JusticePorn/ provides users with interesting legal developments.
http://www.reddit.com/r/AbandonedPorn/ provides a community of explorers a place to share cool photos.
And many, many other sections that I've never seen.
Anyone can create a Subreddit and assign moderators. It's a Darwinian system where the best content (theoretically) makes it to the top with minimal intervention by the moderators; it's based primarily on up voting and down voting by the entire user base.
The disadvantages of Reddit are many; for example, regardless of how cool you think your own story, picture, etc. is, it often gets only a couple of up votes and never appears anywhere significant. Depending on how you sort comments, you only see the ones that occur early on and never see interesting comments that might have been posted well after the story was submitted. Because anyone can submit a link, but links can usually only be submitted once, someone might post a link to a news story before you but the story gets no up votes and so effectively you can't post it. There seems to be a significant desensitization to gore and NSFW posts-- you never know what the link you click on will show when it's just a random URL on imgur.com and could be a horrific injury or death, child porn, disease, surgery, etc. There are sections of that site that you don't even want to know about. Imagine a section devoted exclusively to truly disgusting shock porn, like TubGirl x 1000. It's /r/spacedicks. Trust me, don't ever go there.
Overall, I find Reddit a better site. The news is fresher, the comments are just as good as here, and there are way more comments to be read on Reddit than on the typical stories here. They don't have the "first post" problem because those posts immediately get down voted into oblivion. Also, Reddit has separate sections for people of opposite views on many subjects, such as /r/atheism (constant Christian bashing) vs. /r/Christianity (constant bible thumping). They also have less tech-savvy users, which provides a better cross section of internet users in general and allows better discussion of non-technical subjects. But there's also better information on technical subjects. For example, http://www.reddit.com/r/space/. Or, the time a Canadian astronaut randomly did an AMA (ask me anything) about his visit to the International Space Station, and the result was great discussion on space-related subjects not found anywhere else: http://www.reddit.com/r/IAmA/comments/gm8rh/iaman_astronaut_who_has_been_to_space_twice_and/
Let's be honest, Slashdot is a fading site. They need to do better to compete with Reddit in many areas.