What could be improved for you?

I think it needs a nip and tuck in a few areas, a work of art is never finished! :) for me these are the important areas:

Layout:
Speed improvements to Monte-Carlo radiosity, anti-aliasing, area and linear lights;
Physically accurate lighting models (non-radiosity);
Single frame distributed network rendering.

Modeler:
Better drawing tools for geometric shapes, arcs etc.;
Improved UV creation/editing.

Both:
Faster OpenGL working speed;
Better snapping/layout/measurement tools;
Tighter plug-in integration;
Better on-screen feedback

Some of these I believe have been addressed in the upcoming v8, it's good to know NewTek are listening to their users!

What spec machine(s) are you using it on at the moment?

My machine is a Dual Xeon 2GHz, 1GB RAM, but I'm lucky that in our office we have:
2x Dual Xeon 2.4Ghz, 1GB RAM
Xeon 2.6Ghz, 1GB RAM
Xeon 2.4Ghz, 1GB RAM
Pentium 3Ghz, 1GB RAM

All of which are linked using Screamer Net, the only limitation is that any single frame must be rendered overnight as the machines are used during the day, but sometimes high demand print resolution radiosity renders take priority.

Are there any plug-ins you wouldn't be without?

Commercial:

Rounder by Richard Brak, it's an awesome plug-in that LightWave has needed for some time now, especially for product design work where everything has filleted edges. It's also an great example of how plug-ins should be written!
STL_io, by Mike Wolf. This has helped streamline our workflow from SolidWorks to LightWave enormously, I really can't thank him enough! :)

Free:

DI_PowerTools by David Ikeda, a great set of modelling tools, well written too!
PivotBB/Set Point Position/Align To Rail by Carl Merritt, so simple yet immensely useful!
Normal Move/Normal Rotate by Mark Brown, again very simple yet useful.
R_Bridge/Center_AddP/2P_Info by PICTRIX, absolutely tons of great scripts on his site.
Family Clone by Faulknermano, interface is a bit busy-looking but does the job well! :)
pmodedt.p - by Kevin MacPhail, toggle only between point/poly mode, great little tool!

In your opinion: Integrated or Separated? :)

I can see arguments for both to be honest, I like the fact that not having Modeler loaded means more memory and resources for rendering, but there are times when some of Modeler's toolset would be useful in Layout.
It would have to be done right though, simply chucking Modeler into Layout isn't an option, the interface could suffer.
The other option of course is to have better integration while keeping them separate (if that makes sense!).