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KWin and other window managers supports switching between windows by clicking alt+tab. Also KDE SC allow to switch windows by using special plasmoid).

For me, is not enough. There's still some reason i won't use plasma and won't touch keyboard:
1) Panels are space gainful - I decide to set it to auto-hide
2) We should be able to do every think using mouse
3) Some users would to see similar window(the same role or class) of the same or other application
4) Maybe I won't use plasma

Allow to show window list by clicking button on titlebar or hovering mouse above titlebar by some period of time.

1. If you auto-hide the panel, why can't you just unhide it to change the window?
2. Why?
3. Is this part of the proposal? I am not clear how you would implement this.
4. All major Linux desktop environments have task managers

@TheBlackCat 1. Well, it would force him to move the mouse to a side of the screen everytime he's just trying to switch between applications. It may not sound that bad, but it could be better. The idea to hover above the titlebar for some time sounds WAY more complicated than that though...
2. Why not? Sometimes, I browse the web or edit files using nothing but my Wacom tablet. I wouldn't want to put it away and stretch my hands back to the keyboard just in order to switch an application.

My proposal to Lachu is this: use your desktop corners (there's a setting for that in systemsettings -> Windows behaviour (don't ask my why it is in this category....) -> desktop corners (should be called something like this)) and use the PresentWindows effect which you can see here:
http://www.youtube.com/watch?v=SWaSz4smYlg

I combine it with the very nice tool easystroke, so I'll just have to e.g. draw a line from the left bottom to the right top and all my windows are presented.

This should be what you are looking for (working a lot better than your proposal IMO).

I agree with VaterGarp about the desktop corners. the titlebar for an application doesn't seem the best place to put global operations (switching windows) since it is already full of per-window operations.
Clicking on a titlebar is for activating that window (noop if already activated). Hovering on the titlebar shouldn't invoke an action since there's no way to distinguish between the user leaving/resting the mouse on the titlebar while they interact with the window in other ways (using the keyboard, or just reading/viewing content), excepting unshade on hover when the window is shaded and the unshade on hover option is checked. Other mouse actions on a window titlebar are already slated for different per-window operations.

Compiz does this if your group windows. The effect is really cool, but it is not global. I think we might want to implement that feature into kwin, but it definitely isn't a priority.