Just my $0.02... (1). I think that what Radiant provides in terms of content control to develop and maintain a website is great. It's mixture that developers (like myself) love for its power of customization, and it's not too hard for a non-developer to quickly pick up. If a content editor can't understand page parts and radius tags and how to use them, perhaps they should not be meddling with content. I think by dumb-ing things down too much for your average user will get in the way of Radiant's power users and/or developers.
(2). There I can't agree with you more. From personal experience, the largest drawback for Radiant is lack of page access controls. Yes, there are extensions like members and reader, that allowed us to create our own login system to serve personalized content. But these extensions (and our system based on them) are ugly hacks (e.g. piggy- backing on class-methods to make current_user available in tags, etc). I think page access control (with something like Roles) with personalized content is something that is a must-have for a serious CMS. (3). I have mixed feelings about this. Yes, upgrading should be easier. But if you use a lot of extensions, some of them may not be maintained by authors, and may not work with new Radiant versions, etc. Radiant is a developing CMS, and it's APIs change all the time, so there is a lack of stable API, which makes any meaningful attempts of straight upgrades hard (or even impossible) a lot of the time. At the same time, I feel uneasy letting end-users conduct upgrades to software components. Speaking from experience, I see this as a source of many problems.