We need to develop quite a powerful web application for an investment bank. The bank IT would like us to build it on top of the SharePoint platform
The answer is simple, you should go with what you know. If you prefer to do it in ASP.NET then, that is what you should go with. Trying to learn a new technology on that size of a project will almost certainty cause you severe problems when trying to develop it. Can sharepoint scale to that number of users, probably, but you don't know how to make it do that. That is the real key.
They are correct SharePoint does have a lot of functionality out of the box, but that doesn't mean that it will make you more efficient, because you don't know all of the APIs etc. to access.
Actually, if you want to know the way to cheat. If they force you into using it, you can run ASP.NET applications under SharePoint (well kind of). You can tell SharePoint to essentially ignore a path in the site and use regular ASP.NET as a web application just like any other site does. Really, this isn't using SharePoint, but it can get you out of a bind, in the "Needs to use SharePoint to make them happy scenario".
Mayo suggested contacting MS. I have a feeling they already have a relationship with the bank and have provided some insight about the project. I would contact: http://www.mindsharp.com/ and see if they can help you out. They are a training company, but I bet that the owners would be willing to help consult, and I haven't found anyone with more knowledge on SharePoint than Todd Bleeker.
UPDATE: After looking at this more I would add that I do not believe that SharePoint is for you. As I mention below SharePoint is for collaboration. If the users that come to the site require an isolated experience then SharePoint is more overhead than you need.
SharePoint is built on top of ASP.NET so you have everything that you want to do with ASP.NET in addition to what SharePoint provides. Anyone who says that it is difficult is trying to make it that way. You can deploy stand alone custom pages with 100% of your own code and it will run under sharepoint, or you can create new application pages that also contain any code you want to write, or you can simply add your own webparts that can be added to any page you choose with 100% of your own code.
Here is just one example.
Creating an Application Page in Windows SharePoint Services 3.0
What SharePoint offers on top of that is a whole different paradigm on collaboration tools. If you wish to leverage it (if not the cost on return is somewhat limited) you can build amazingly complex and integrated solutions that is build around the aggregation of data from across an enterprise.
That being said, do not go into it lightly. If deployed wrong or with a half understanding of where SharePoint excels and where it does not will result in a diaster. Unless you have the time to understand the core concepts of SharePoint I would warn against it but your client is right. If you do build it in SharePoint you get a great deal more flexibility. One right off the bat is the ability to mix authentication modes. I designed a solution that mixed custom forms authentication with an LDAP backend with Windows Authentication. Anyone could visit the same pages but your authenticated account could come from two different locations.
I would build this on SharePoint. It is quite suitable for big sites and many sites have already been built on it: topsharepoint.com
SharePoint (like all complex applications) does require sufficient knowledge that you do not seem to have at the moment which is a big risk in my mind. Don't listen to the nay-sayers though.. lack of knowledge is a common problem for devs dealing with SharePoint but it doesn't mean you can't make it do whatever you want.
Regardless, what other options do you have? I think the days of building completely custom CMS's have passed just as building completely custom Intranets are not cost effective anymore. There are many competitors to what they want to do with SharePoint (Umbraco, Sitecore, Sitefinity, etc) and most of them seem better than 100% custom.
So the answer might be neither ASP.NET or Sharepoint..
Do you need document management? Do you need version management? Do you need to create "sites"? Do you need audience filtering? Do you need ECM (fancy word for CMS), Do you need collaboration stuff on your site? If your answer is no then SharePoint is not for you.
You said "We have never made any SharePoint programming" and for that reason alone I think you should not use SharePoint. You also say that your app is going to be "non-standard" and complex, another reason not to use SharePoint.
Sounds like you know ASP.NET so I would advice to stick with ASP.NET or ASP.NET MVC.
Hope this helps
What a load of CRAP that sharepoint isn't cut out for what the op wants to use it for. Especially the "Do not get yourself wrapped up in SharePoint" comment from ChaosPandion. Maybe he thought it to difficult and gave up...
Sure SharePoint development takes some getting used to, but it is able to what is wanted by the op most definately. SharePoint is built using ASP.NET so anything you do in ASP.NET can be used / ported to SharePoint. It is not a standalone product, but a DEVELOPMENT PLATFORM. It will scale to serve that many users, using multiple WFE's (Web Front Ends) and a SQL Cluster as backend.
The question here is: is sharepoint the most suited platform for building this site? Then I would have to answer, probably not, seeing as the wanted functionality is almost all custom development. If you plan on doing web content management as well, then yes, SharePoint is definately worth looking into. Also, SharePoint takes away all (or at least most :-D) authorisation and authentication wories. It is Department of Defense certified. And if the offered out of the box security is not enough, just write an authentication provider (seeing as SharePoint uses ASP.NET's provider model).
To answer your questions:
The bank IT told us that SharePoint has in fact lots of other features that will help us make the project more efficiently - for example, it seems that SharePoint has some built-in scalability and high availability technologies.
SharePoint is farm based, to which you can add machines, having each machine perform a different task, which means either app server, index server, WFE, document conversion services., WFE's can be behind a load balancer to distribute requests. Also I want to mention the web content management again.
I heard saying that SharePoint development is very tedious, that the platform cannot be very easily customized, etc.
Like I said, SharePoint is based on ASP.NET, so it is as much customizable as ASP.NET is. You could even create an ASP.NET web site, put all UI in Controls and then use those is SharePoint, maybe even have the controls use it's own database. As for it being tedious, not really. It's just DIFFERENT and deployment / testing is not like normal deployment / testing. SharePoint uses so called solution files (.wsp files), to package up functionality and deploy it to the server. This IMHO makes it possible to deploy functionality in a very modular way. Furthermore, there are loads of cool open source projects out there that make sharepoint development much easier and also provide cool extensions to "pimp" your site and make it more fun and easy to use for end-users.
Nuff said....
You said,
I heard saying that SharePoint development is very tedious, that the platform cannot be very easily customized, etc.
You have been misinformed about SharePoint. All SharePoint pages are ASP.NET pages. You can customize any of them, either directly, or by using Microsoft Office SharePoint Designer, which is free.
Get started at http://msdn.microsoft.com/en-us/sharepoint/default.aspx.