First off, 4.0 is amazing and there's so much promise in this system now. I've made up my mind to abandon Traktor and move to RB. I love it. Cleanest and most intuitive user interface, Retina support, Related Tracks feature, and of course plug-and-play with Pioneer gear. I'm really excited about all of it.
In my play-testing this weekend, there are definitely some missing features and a few bugs that I'm sure the team is probably aware of, but I wanted to mention them here just in case. I'm playing on a DDJ-SZ with firmware 1.20 and the latest drivers.
1) Key lock. EDIT: I'm an idiot. Key lock is Master Tempo, and it works brilliantly.
2) EDIT: Engineering is aware of this issue. Quantize and cue point issue. There's something seriously wrong with using Hot Cues and Quantize in conjunction. I'm not using any kind of sync, yet very often, when I hit a Cue point, it will trigger an entire beat late. Surely, this one is already on your radar.
3) Color coding and harmonic sorting order for Keys in browser view. Other DJ softwares include this feature. I'd like to see my keys in sharps or flats, but still be able to sort my tunes by the key's harmonic order. Right now, sorting by key is alphanumeric. I guess the assumption here is that most folks use Open Key notation, but I personally do not. And in either case, color coding of the key would help tremendously, too. Also, correct me if I'm wrong, but having RB truly understand the 3 popular key notations in use today would greatly improve the accuracy of the Related Tracks feature as well. Color coding keys in the deck view would also help them stand out at a glance.
4) No way to edit track meta data. I tried in both Performance and Export mode, but I couldn't find a way to e.g. change a track's star rating. As far as I understand, this field is read on import from iTunes and then duplicated in Rekordbox since there is no ID3 field for it. That's all well and good, but previous versions of RB allowed the user to change the rating.
5) EDIT: Apparently editing the beat grid in Performance mode is possible by clicking on the deck number. Cool! The ability to edit the beat-grid while in performance mode would be much appreciated. As users adopt RB, it's really painful to sit there and grid all of your tracks in one go. It'd be significantly easier if I could grid tracks as I loaded them up to play. I realize that this is a fairly significant UI change, which may impact Pioneer's ability to get to this one.
6) The ability to remove a Track from your Collection while browsing the iTunes node. It's tedious to see something in your iTunes node and wish to remove it from RB, but have to then go find it again under the Collection tree to do so. Please enable the right-click menu option to remove tracks from the Collection regardless of where the user happens to be browsing.
7) EDIT: Possibly slated for roadmap, but no guarantee. The ability to change controller mappings. I suppose Pioneer is attempting to lock down the set of controllers that work with RB. But even for supported controllers, being able to tweak or tune the mapping would be valuable. For example, I will rarely use the Sampler performance pads on my DDJ-SZ. I do, however, use the Beat Jump feature extensively -- yet Beat Jump is mapped to Shift + Hot Cue. I'd love to assign Beat Jump to the Sampler button (which would also be more consistent with RB's UI, btw). Again, this is another huge UI request, I get it..
8) Browser search function should ALWAYS be accessible from Cmd+F / Ctrl+F. This one is pretty self-explanatory, but a savvy user should never have to click on the browser search bar during a performance. The platform-appropriate hotkey shortcut should always bring focus to the search box, from anywhere.
9) EDIT: Preview will work on the DDJ-SZ if RB is set to Internal mixing mode. I kind of assumed that the Preview feature would Just Work(tm) with my DDJ-SZ, but it doesn't. What's worse, is that none of the available outputs in the audio mapping preferences in RB allow me to target the headphone output. Is this a limitation of the hardware (I'm new to the SZ), or RB, or am I missing a simple preference setting to get Preview coming through the headphones?
This sounds like a lot of harping, but in truth, I had _so_ much fun playing on RB this weekend, and like I mentioned above, there's tons of promise in this software. Kudos to Pioneer and the RB team for what they've done so far. Please keep the updates rolling!