You have an idea.  Will it work?  The easiest way to find out is to give it a shot.  Too expensive?  Then, prototype it.  Create a simpler (read: cheaper) version and see if your potential users (or customers) will utilize it the way you hypothesize.

This is usually the advice I give to entrepreneurs if I am meeting them at the business idea stage.  However, as Tim Levine of SocialMedia.com points out in his post at Inside Facebook, this could be a tricky approach.

The most common mistake in A/B testing is not running enough ‘trials.’
The second is running too many. The latter risks wasting potentially
more productive opportunities, but the former is far worse because you
risk managing by noise. How wasteful it would be to invest in a
redesign because of a difference that might just be from random
variation!

Many internet ventures in Turkey depend on, and make assumptions regarding, CPC advertising.  Turkish entrepreneurs should find the example Tim gives in his post useful.

Ryan Singer of 37signals blogged on the recent screw-up around profiles at Netflix:

Here’s another reason to double, triple, quadruple-check yourself
when you want to add a new feature. A while back Netflix added a
“Profiles” feature to their service. A couple weeks ago, they decided
to pull the feature
because it was too confusing and it wasn’t adding value. But it was too
late. People were pissed. The blog post received 1286 comments. In the
face of this reaction, Netflix had to turn 180 and keep the feature. Whether Netflix Profiles are good or bad, clear or confusing, they’re here to stay.

The
lesson: Once your user base has grown beyond a certain point, you
cannot take features away from them. They will freak out. Whether the
feature is good or bad, once you launch it you’ve married it. This
changes the economics of feature additions. If you can’t destroy what
you build, each addition holds the threat of clutter. Empty pixels and
free space where a new feature could be added are the most valuable real estate on your app. Don’t be quick to sell it, because you can never get it back.

I quoted the entire post since I think this is a very very important issue.

The temptation to add features if your product is not seeing the uptake you’d hoped for in the market is a big one.  Add to this the pressure that your competitors are putting on you through press releases on new features.  Then you’ve got your salespeople telling you that they keep hearing demands and desires from prospects…  It adds up, you give in, and pop comes a new feature, along with the baggage Ryan describes above.

I think it’s exceedingly rare that a web application’s success or failure is related to whether it’s feature-rich enough.