Yeah that copper is Vcc and Vee. I can obviously convert all of those fills back to traces (they were at one point). I figured pouring Vcc and Vee would lower the impedance (series inductance) and would be a good thing. Is this bad from an EMC standpoint? I've seen this done..but I would like to know why it might be (is) bad. Are you suggesting to fill the top layer (whatever's left after routing signals and power) with GND? Better for conducted emissions..?
Ah. Yeah-- that's not too bad if you've placed a bypass cap at every "peninsula" of Vcc (which, actually, would make the layout almost as good as a 4-layer board!), but it seems likely that there are a lot of those, and so, placing all that bypass would be more irritating than bypassing only where needed (at the ends of Vcc traces).
If you've done the one but not the other (i.e., bypass at pins, not at the tips of peninsulas), that's certainly still better than otherwise. The downside is, you don't have any idea what kind of impedance and resonance that supply will have, unless you take a solid look at where all the copper is. (And, as your layout changes in development, the negative space around that routing also changes, which means your Vcc network changes!)
Any cuts, slots, gaps, peninsulas, islands, stuff like that, are potential antennas. From any such feature, you get a few peaks at high frequencies (where the wavelength is on par with the slot length or island perimeter), and an equivalent inductance at low frequencies. If you have a lot of current (like a switching converter), the equivalent inductance will be important.
An antenna is symmetrically a receiver and a transmitter. If your circuit is producing noise at frequencies where the antenna resonates, then that's an emissions problem. If your circuit is sensitive to noise at frequencies where the antenna resonates, then that's a susceptibility problem.
The latter is probably the bigger concern (especially with a circuit that doesn't have any obvious super-high-frequency sources!). Especially as RF can be rectified into DC offsets or AC crosstalk.
Have you ever had the experience of a sound system producing a "bip bip beep" sort of sound, just before receiving a text message or phone call? Yeah, that's GSM radio being detected by a shitty analog circuit!
Even if the circuit isn't an emission or susceptibility problem, this is good practice for the cases when it is (anything switching, digital, high frequency, high sensitivity). And making it better, even when it's not
required, will still make the circuit strictly better (though maybe not by enough to measure..).
Tim