I made a post on Ubuntuforums in this thread, listing my views on what could be improved Thunderbird-wise. I was going to make a short point, but the list grew. So I thought I might as well post it here too:
If Thunderbird doesn’t do RSS or usenet good enough , then why use RSS and usenet with Thunderbird? Albeit, RSS and usenet are both somewhat similar to e-mail (you get “messages”), it is not e-mail, and isn’t necessarily something that even should be handled with your e-mail client to begin with.
Anyway, I use Thunderbird, and my annoyances are these:
1. It doesn’t look very good. There are Human & Tango themes for Firefox in the Ubunto repo. They’re easy enough to make, so why not make some for Thunderbird? 
2. It doesn’t have a calendar. Lightning is crawling in terms of development, and it doesn’t even come close to Evolution calendar anyway. Why not make a Thunderbird-[Evolution-calendar] plugin, making the two communicate on the following aspects: unified adressbook, unified calendar, unified theming/skinning/whatever? Splitting Evolution into several modules would ease this.
3. It doesn’t have a lot of extensions either. There are 350-something listed on addons, but many of these are old, or for Windows and/or Mac only. In Firefox there are an insane amount of extensions. I don’t really know what I can’t live without in Thunderbird, but there are areas that could be covered. (I’m serious here: When I look at the list of Firefox extensions I think: a. How do people come up with this stuff?, and b. How did I live without it? I need this experience with Thunderbird. Like split browser and showcase in Firefox.)
4. It doesn’t integrate well with Gnome/Nautilus/whatever (This is a crucial point)
5. It’s slow in development compared to Firefox. Really slow.
Now, the last point covers many applications besides Thunderbird:
6. Apps in general are starting to look old. Compared to the current development in eye candy, where are the eye-candied icons (like all things tango-ed), the rounded edges (inside windows, the window decorator takes care of the oustides)? If you don’t understand what I’m talking about, take a look at the apps mentioned in this post: https://anotherugly.wordpress.com/200…mes-for-linux/
There is a solution to this: A lot of apps are skinnable/themable in some way. Why not make a deb called eyecandy-all that sets up all applications to look better? Some extensions for Firefox and Thunderbird, the change widgets for Firefox stuff, making Wine look better with a better theme, making KDE look Gnome-ish, etc. All these mentioned here, at the top of my hat, are featured on Ubuntuforums already, but they take a lot of time, and could easily be handled in one easy deb.
To me, there is no alternative to Thunderbird. Evolution lacks so much, and there isn’t even an easy way to import all my Thunderbird stuff into Evolution. But Thunderbird has a long way to go…
 What excactly are the grievances with NNTP and RSS on Thunderbird anyway? What are the tools you’re lacking? I’ve been using news with Thunderbird since 0.5 (or thereabouts) and RSS since it was implemented, and think it works fine. I actually prefer the RSS setup with Thunderbird over Liferea. (Using Liferea on my multimedia-box, where I don’t need e-mail, in order to get some basic handling on music-relevant RSS-feeds. (Like Dime)
 I would try myself, but I suck at drawing…