Plenty of people submit worthwhile things that just sit at the firehose, while plenty of shilled crap gets front paged.
Unfortunately, everybody's definition of "worthwhile things" is different. If there are specific examples you'd like to discuss, I'd be happy to post my perspective on why they may or may not have been posted. I realize that submitting to Slashdot can be like screaming into the void; it's something I always wanted to change. But there are often good reasons why submissions were declined.
Shilled stories get to the front page for a few reasons. Here's how CmdrTaco explained it to me when I joined Slashdot. The role of Slashdot is to be a filter; it whittles down the tens of thousands of articles posted every day to the 20-30 that are most relevant to the community. Most publications pump out lots of junk every day, along with a legitimately good article or two. If a shill wants to submit their best article for consideration... fine. Non-shills are preferred, of course, but more on-topic submissions in the firehose can only help the community. Editors can (and do) reject shills all the time. Even StartsWithABang only has a ~23% acceptance rate.
I dare you to explain how it is decided which things are front paged, including the names of the people who make the decisions.
And I defy you to directly state that you still work for Slashdot and that advertising or other promotional deals do not affect what is posted to the front page.
I haven't worked for Slashdot since the acquisition. I can directly state that for the duration of my time there (Dec. 2007 until Jan. 2016) no advertising or promotional deals affected what was posted to the front page. (Excepting, of course, these dumb things that started a few months ago, which were straight up ad units, and labeled as such).
The editorial staff decides what gets posted. Prior to the acquisition, it was myself, samzenpus, and timothy. Since the acquisition, it appears to be timothy and a new editor named yaelk (and occasionally whipslash, one of Slashdot's new owners).
Stories are picked using a variety of criteria: how the community votes on it, how interesting it is, how on-topic it is, its relevance, the quality of the source, the article's timeliness, what similar material is on the page already, and a few other things. (Disclaimer: I am speaking for myself, and how I picked stories, but samzenpus and timothy operated similarly -- as CmdrTaco taught us).
These criteria are weighed against each other. If an article is a few days old, it needs to be particularly interesting to make the front page. The more off-topic something is, the more interesting it needs to be to make the cut. How the community votes is important, but is not enough on its own. The community sometimes votes for things that are factually untrue, or are years old, or involve attacking somebody. The community sometimes votes up dupes. On the other side of things, sometimes the community just doesn't vote.
Hope this provides some context for you.