I’ve been using trackers since 1992. All this was of course on Amiga, so I used mostly ProTracker and later DigiBooster (essentialy a ft2 for Amiga). When I switched to PC around 2000. I tried using fasttracker2 but it was already a derelict piece of software, so I was stuck with running protracker in WinUAE untill Renoise came along.
I’ve only been fiddling with it for a while, so I’m avoiding automation and controlling VST and most of the stuff that is not familiar to me. What I immediately grasped were the instruments editor and sends. I still have no idea why the ‘S’ track exists, or what half of meta devices or tabs do. Once I’ll just click on every button to see what happens 
GRIPES:
Considering my background, the hardest bit was getting used to sequencer commands that have changed from ST/NT/PT/FT standards. It would be neat if there was a quick reference somewhere within Renoise. Say, a popup under Help menu, preferebly with a key shortcut as well.
Meanwhile maybe we should make a series of reference-card like images that can be set as wallpapers?
Second gripe I have is the awful tiny window for samples of an instrument. It is as if looking through a tiny hole into a vast space, and I really see no justification for this, even though FT made this a ‘standard’. I understand there are GUI space issues, but it can be solved as a popup/foldout thingie. IMO.
Third gripe is the on-screen keyboard. First, and not really important, it just floats there in emptiness and that neither looks good nor feels good considering how the rest of the interface is packed with info and features. Second thing about it is that you can’t really always tell when a black key is playing, and that’s really not that hard to solve. I don’t use a MIDI keyboard, and I like the visual feedback that trackers give you (like seeing all tracks side-by-side, or seeing where on a keyboard is a note that is playing) and this in my opinion adds a lot to the sensation that you are in control of all the data that constitutes you music.
Maybe just make black keys more obvious when pressed. Maybe make the keyboard a bit scalable (or at least have a larger and smaller version depending on resolution).
While we are at it, I really liked (and used) the fake-analyzer that protracker had. It just displayed the notes being played, not their actual frequencies, but this was very useful to see on the fly cause you then saw where there are gaps to put more notes or instrument. This to me is as important as being able to see all the tracks playing side by side and SEE the gaps in rhythm. I guess what I’m trying to say is that I’d like to see more visual feedback from the song as it plays.
WHAT I’D LIKE TO SEE:
Apart from the obvious stuff like VST and effects and whatnot, I think the biggest heritage of Renoise is that you can rearrange tracks or patterns or instruments by dragging and you can rename them and such. This is very useful for organizing your song. I’d like to see more stuff like this in the future. Perhaps selecting then shift-dragging in arranger, or being able to group tracks or color-code tracks or instruments or patterns.
The mixer made track effects overview and navigation much easier, but this is still the biggest workflow killer for me. I don’t really have an idea how to improve this, except maybe folding/unfolding dsp’s. While we’re at it, I’m not a piano roll whore, but I sure as hell a ‘machine/routing view’ whore (like in Buzz/Psycle/Aodix/AeroStudio). It’s the only reason that I waste some of my time in Psycle instead of concentrating on renoise but I understand that because of the way the sound is organized it is impossible to implement such a visual organizer in renoise. Oh well. It would be just too perfect, I guess 
Now I’m going to the other room to drop dead from all this typing. 