I think this is a needed feature.

I'm surprised not more votes get to this. Maybe the description is too short - and Tree View is not really a title that helps non-technical people to immediately understand what's being asked for.

So, while it has been explained below already, for the sake of people coming here trying to get an idea of what this request is about:

Gnucash should look similar to the Windows Explorer. Where in the Windows explorer you see the strcuture of drivers and folders, in GnuCash you would have your Account hierachy. Where in Windows Explorer you'd see the content of a folder, you'd see in GNuCash the transaction of a selected account. By clicking on an account, the content windows would immediately switch to the new account.

Yes, this would be a very helpfull feat.
I think only reports etc. should open new tab, or a option in the context menu "Open Account in New Tab". Otherwise, clicking in the tree should just do what was described: Opening the tree and displaying the selected account in the right.

I agree with the point about there being no "Quicken" alternative in OSS. It seems that artwork and intuitiveness (if that's a real word) is lacking in OSS. Maybe someone with some artistic ability could volunteer some time to make this a little more polished looking?

I guess it is sort of what Charles said. Open Source programs always suffer from one of about four issues. Either they don't have great art work and therefore look ugly (who wants to use ugly programs?), they start out amazing then because of lack of support they die, they aren't as feature rich or intuitive as their paid counterparts or they just lack funding.

I think the only two issues with Gnucash are the intuitive/artwork issue. I understand that mostly number crunchers use it and don't need the extra bling... but Gnucash is competing with the likes of Quicken and such and if I was a CEO, I probably would see quicken as the only option.

And yes, tabs on the side are still tabs.

Not exactly what you ask, but you can choose to put the tabs on the left side already now via the preferences.
I know, I know, that's not the same as a tree view, but perhaps it reduces your current issue.
Other than that, I have no opinion on a tree view vs tabbed view. I have some difficulties imagining how such a tree view could be set up in a sensible way for gnucash.

I like this idea too. I think it is the one main feature to making this program much like a Quicken product. (Besides the ugly artwork).

I'm all for this feature!

If what I said is unclear, it is like an "explorer" interface. Folders on the left, content on the right. It makes everything so much simpler to navigate and you don't end up having 300 tabs open all the time.