> I like tikiwiki for its features. No doubut its a great CMS. But I was wondering whether this can be used for developing a robust/stable corporate level intranet with say employees over 15000?
Yes. Tiki is suitable for such a project. But as with any advanced system, there is a learning curve.
To have just personal blog, Tiki would work but is overkill and it is not always worth the learning curve. But with 15 000 users, for sure you should take the time to learn and configure a full-featured CMS/Groupware such as Tiki. For 15 000 users, you will want a powerful user, group & permission system, which Tiki excels at.
For 15 000 users, you will want to have a lot of features you can activate with a few clicks. For sure, one day, someone is going to come asking for a new feature. Imagine if you install 2-4 systems to get all the features. You will need to synchronize userbase/menus/themes in 2-4 systems. Yuck!
I agree that Tiki is not as intuitive as it should be. The code & features are a lot better than the documentation. But that is something which is being worked on. And it is something you can participate to. -> Doc.tikiwiki.org
Already, in Tiki 1.9.4 several of your points have been addressed:
Tiki 1.9.4 includes a lot of tips in the admin panel to learn how to do things.
Tiki 1.9.4 now includes by default the quick_edit module to create a wiki page.
Permission management has been simplified by default (and old way is still available by clicking "advanced feature"
Page vanish? I manage 40+ Tiki projects and I have never had that problem. Check tiki-listpages.php