Updated: 06.12.2010
I've compiled a list of some suggestions for latest beta, decided to share. It's a short notes, some lines probably need a better explanation, ask if something is unclear

I. UI
1. Interface templates - like ones in adobe after effects and photoshop
2. Unified mouse navigation in all panes, compatible with viewport keys
3. Shift-click to quickly collapse/expand sub-trees of materials
4. Copy/paste of nodes(and maybe parameters)
5. Color swatch - hide it when mouse is outside of swatch for more than 1s(maya2011 behavior)
6. HSV mode for color swatch
7. Mouse wheel should only scroll up/down right pane, and not be used to change parameters to avoid of accidental change of random parameters
8. Text file for keybindings like in after effects(not necessary, but would be nice)
9. More compact UI(Like Modo or LW), but at the same time - bigger knobs for values, those triangles is too small(imagine people working with tablets).
10. Ctrl-clicking control knobs could work as a "0.1 scale" modifier(After effects)
11. Would be nice to borrow Houdini behavior for pop-up menu - with "Tab" button and typing first letters into it to quickly creating things.
II. Viewports
1. AF/Material picking tools could be mapped onto mouse buttons like Photoshop tools - with alt-ctrl-shift modifiers
2. Exposure/F-stop controls right on viewport, shown/hide by the same icon(AE/Nuke/Maxwell)
3. Fit selected/all/switch to cam buttons is a must
4. Softimage/XSI has four quick viewport bookmarks right on each viewport window - with, say, left mouse button you can save current camera position, with right/middle click you can recall it. Would be nice to have those in octane.
5. Some interface to quickly apply overlay text onto rendered image with render stats in it(iirc vray got the same feature built into their own render window), like: "xxx0001.obj, triangles: 10000, render time: 600s, samples: 5000, materials: 26", etc, etc. Information to display could be customized with some predefined checkboxes, or something.
III. Materials
1. Materials library - ability to save collections of materials, or separate materials into a single file
3dsmax material libraries is a good example, maybe with textures integrated.
2. Cosine/falloff function
3. More test render objects - plane/torus/cylinder/cloth/etc/etc
4. Materials should contain built-in swatches so user could quickly select needed material in library
5. Would be nice to be able to extract/mark parts of mesh(based on connectivity maybe) and assign another material to those parts, few times i had situations when i wanted to assign different materials to different parts of single mesh and reimport either was bugged or not possible for some reasons.
6. When picking materials in viewport it's plug on mesh node should be highlighted - in big scenes with dozens of materials it's realy PITA to find particular material plug on the node.
7. Would be nice to be able to drag'n'drop materials right onto viewport. Or it could be a tool similar to AF picker named "apply current material tool" or something like that.
8. Would be nice to be able to extract those mesh materials into a separate nodes to quickly rewire/change them.
9. Just as an idea: you could implement something like Maya "isolate selection" mode, just based on materials, something like "isolate objects with material". Sometime it could be useful for quickly tuning scenes with lots of mats/objects.
IV. Render
1. Option to save rendered image after some time intervals
2. Option to lock viewport to avoid of accidental rerenders
3. Rough render time prediction based on time passed and current samples
4. Usually when user switching between directlight/pathtracing, he has to change exposure as well,
would be nice to keep two values of these and automatically switch between them on kernel change
V. System
1. Some custom file format with open specification to avoid obj usage(RIB?)
2. RIB support(including archives and some advances stuff)
3. Ability to collect and save all the files in current project into single folder(and maybe autozip them)
4. System "forgetting" previously set values in materials when switching to another type of control
and returning back(float-RGB-float = first float lost)
5. System wide undo?
6. What about tighter integration into major 3d packages?
7. Command-line version, is it possible? Would be nice for animations/etc.
VI. Live DB
1. While online live DB is a really good idea, i think it should have few additional display/sorting modes:
by author, by type(type/category could be incorporated right into material, and let user select from available types or create a new one - "cloth", "concrete", "asphalt", "stone", "glass", "organic", "non-organic", etc). Under "different sorting modes" i mean that the whole Live DB tree could be a kind of a "virtual structure" dependable on current sorting mode: if it's "type of material", we will see "stone", "glass" sub-sections in the tree, if sorting is set to author, we will see "Radiance", "Joss", "unknown" in the DB tree. Or, for example, you can implement sorting by material opacity if you want. Sorting by any material parameter, that's what i mean.
VII. Bugs:
1. When user clicking mesh node in graph editor, then collapsing all the materials in node inspector with "collapse all nodes" button, then expanding the only needed material and changing it's type from, say, glossy to diffuse - all materials suddenly expanding back.