“User-Q”: Sadly there is no user on SU with your name so your comment doesn’t sound very trustworthy. Indeed it’s mostly not true as well.
Especially this sentence
“If you don’t create worthwhile content, no one will support you just so you’ll do the same with their own worthless content. However, good content is vastly rewarded.”
is rather what we wish would happen but in reality SU is full of low level content theft “fun blogs” serving “creative images” stolen from elsewhere. Just check the most popular submissions.
Also suggesting that I post spam on SU or I am dealing with people who submit spam disqualifies your comment additionally.
Nonetheless there is one truth in your comment, the underlying disdain for and hate of anything business. While SU has alienated businesses Twitter will succeed due to embracing business users. SU can’t compete by copying Google or Twitter from th eoutside without the value those two produce.

People stumble their own pages all the time, despite what you say above. If the only reason you log on to SU is to post spam, you won’t get far, which is a good thing for everyone. It’s a good system for generating valuable hits and attention, which old-school views-per-million marketers will never be able to understand or capitalize on.

First, the categories aren’t prefect – but they are much better for an international audience than many other services. Look at Digg’s categories for example and then imagine you are a non-American looking for non-American stories. That just doesn’t fit with how they structure their categories.

Second, my experience of British sites is that the level of traffic from social network sites is often extremely low, if any. However, StumbleUpon is the most frequent ocassional exception to the rule.