In my previous entries I've mulled over the statistical effects of a few pieces of fan art, but I've never properly discussed the obstacles facing a webcartoonist.

The first problem is creating content other people want to see and this is no small challenge.  Aside from conjecture there's no way to gauge just how big your potential audience is.  For example gamer comics have massive popularity, Penny Arcade, CRTL+ALT+DELETE, but that market is over saturated.  Likewise super hero comics are still popular, though their audience has been sharply declining as a greater variety of products have become available, and that market is also over saturated.
Ryan North has managed to launch a career from Dinosaurs Comic, a comic strip where he has used only one piece of artwork for years, but changes the dialogue every time. His work has never appealed to me but commercially it has been a smashing success.  The Oatmeal by Matthew Inman is a huge success in terms of audience and financial gain even though the comic is nothing unique.  It combine Gen Y humor sensibilities, with very basic principles of comedy (ie craft jokes about experiences and point of view your audience will likely share) and artwork that's decent but nothing exceptional.  Saturday Morning Breakfast Cereal is extremely successful and initially it started out as nothing more than a weirder than usual odd humor strip in the same vein as the Far Side and Bizarro.  The Foglios have somehow managed to make a living out of doing Girl Genius a product under normal circumstances would be consider to appeal to such a small niche very few editors I think would have ever published it.  There's also PHD by Jorge Cham which is just a really well promoted slice of life comic.

What I'm trying to say is there is no easy way to predict what will be successful.  There are lessons you can learn from those who have succeeded.  
1)Unless you happen to be on the cutting edge of a popular trend the odds of breaking into and establish market are slim because you have to establish yourself amongst a pile of people doing the exact same thing.
2)Generic content if executed well enough and effectively promoted can always find an audience.
3) The internet is fabulous home for niche content, because the audience can more easily find it than in the preonline world.

In otherwords if your content is decent enough you can find an audience.

The greatest challenge facing an online cartoonist is not content but acquiring an audience which is the inverse of the way cartooning used to work.

I'm nearly 30 and back when I was growing up cartooning was centered around printed publications.  Your content had to deemed worthy by and editorial staff at a syndicate or some magazine or newspaper of being published.  These editors acted as a filter and determined what content was viewed by their audience.  In some ways it set a certain floor to the quality of work that was made available to the general public.  But it also kept a lot of content out.  Material put before the general public at large must be "family friendly".  Essentially what "family friendly" means is that the work had to appeal to a  slightly socially conservative middle class white demographic.

Once you got in you were guaranteed at least a shot at putting your work before an audience, but the fate of your career was strictly in the hands of editors.  Your work could be cut before it had a chance to grow and develop, or as is the case most commonly nowadays newspapers keep dated formerly popular content on life support so as not to offend their dwindling aging subscriber base.

This was a stable system that has lasted for around a century and is rapidly crumbling as the general public continues to abandon printed content.

Now the world of cartooning is a giant free for all.  You have to give the audience a reason to follow your work rather than the all the other content online.  You are not in competition against just the other web cartoonists.  You are up against the entire world.  You have to have something that makes people look at your work rather than the tens of billions of snippets of entertainment online.  More importantly you have to find a way to place your content in front of people so they actually notice it exists.

The greatest challenge of web cartooning is getting people to know you exist and in the beginning no one will help you.