On the first, you should ALWAYS include full text in the feed. Why? Because you'll get far less traffic if bloggers don't read you and don't link to you. We can send a ton of Google traffic your way (three links from "A list" bloggers will guarantee you first page appearances). The feed that does it worst? Microsoft's own Slate. I unsubscribed and will never visit their HTML (and, you never see me link to them, do you?)
On the second, feed producers should ALWAYS leave the reader in control. Please, no special fonts, no special colored backgrounds, no CSS, no branding. Thank you. Why? It's easier to read. Imagine if the New York Times put a different font on each article.
On the third, this would take care of itself if news aggregators had the ability to automatically sense whether or not there's a feed and present the choices to me in some sort of UI.
On the fourth, I hate it when webloggers don't have their syndication feeds as an XML icon. The XML icon is easy to find, and its behavior is easy to learn. NewsGator lets me right-click on any XML icon and choose "subscribe."
That third one would be a lot easier if feeds were presented with their own url form - something like the feed:// form that has been proposed. That way, aggregators could simply be registered as the "default" handler for that url type. I'm not seeing any movement on that front though. The second point is one I hadn't thought of, but it's a good one - pick up any news magazine or newspaper. Does the font and background color vary? I'm thinking that web designers still have a lot to learn from the print business here.
As to full content feeds - I agree with that completely. I understand the desire to pull people to your site, but - I'm noticing that over time, I read fewer and fewer sites that don't include full content. Yes, that's clearly anecdotal. However, going to the site is an extra step - and certainly more work than just moving along to the next new item in my aggregator....