Complex problems need complex UI to do them justice, it's true.

But it doesn't change the fact that complex UI is hard for users to learn.

If users only use your software rarely, instead of daily, they will not be capable of learning a complex UI.

If users don't know or think of what they are doing as complex or at least important, they will not be willing to learn a complex UI.

If your software started out with a clean simple UI but added features little by little, and many/most of your users have been with you all along -- that was a lot easier to learn, because they learned it little by little, and don't even realize how complex the thing they learned is.

(This also means if you can hide complexity for beginning users and add it in little by little, this can help it's learnability. But easier said than done -- Microsoft Office's attempts to hide 'less used' menu items is an example of failure that made things worse).

A shopping site with a complicated UI is going to fail miserably -- unless you manage to make a shopping site that convinces users it really does something much much better than the competition, and/or a shopping site your users will use daily.

Photoshop is something users use daily, often as part of their paid job, to do something the users recognize is fairly complex work and find valuable -- AND many of it's users have probably been using it since it was simpler and learning it little by little. Newcomers to photoshop (or gimp), especially those not using it for a paid job, often find it very difficult to learn and give up.

Facebook is something users use daily, for something they find indispensable. It also does an _okay_ job of providing a UI where the users can ignore _most_ of it until they need it (and there are many parts that many users have never found even if they would be useful). It's also got the network effect lockin -- a Facebook competitor with a "better" UI would have trouble competing even if it _did_ manage to come up with a just-as-powerful-but-simpler UI, assuming that were possible.

Craigslist -- I think actually _is_ a clean/simple interface. Better graphic design styling probably wouldn't hurt it (I really don't think it would, in any way, if done well), but it's not complicated.

It's true that powerful functionality is very hard, or sometimes impossible, to do with a clean simple UI. And powerful functionality is great. But it's also true, I think, that no matter how powerful your functionality, users are going to find a complex UI difficult, and they have to have a lot of motivation to learn it.

For a shopping site someone uses only once or occasionally, I am still convinced clean simple UI is always going to 'win' (increase sales, get more sales and users than it's competitors all things being equal, etc).

I am also convinced that you should always _aspire_ to as clean and simple an interface as possible. If you can get powerful functionality _with_ a clean interface, users will always like that better than the same func with a complicated interface. (People _use_ MS Word, but few _like_ it. If someone could figure out how to provide the same func with a cleaner interface, people would go wild for it. It's a big 'if', it's not easy). If you can't figure out how to do that (and it's not easy) -- it is hard to predict whether less powerful cleaner software or more powerful complex software will 'win' with the users in any given case, it just depends.