Palmer covers this pretty well in 8.3 of How to Brew. I'll be interested to see if his new revision changes any of this description, but the key points make sense to me:
1) Even though most yeast has settled out at this point, there is still plenty of active yeast that is engaged in things like reducing fermentation byproducts (acetaldehyde, esters, amino acids, ketones- diacetyl, pentanedione, dimethyl sulfide) and converting fusel alcohols into other compounds. The presence (and conversion) of these will produce changes in flavor.
2) High molecular weight proteins and tannin/phenol compounds will bind together and settle out.
Also Palmer doesn't mention it in this section, but I find that the yeast itself has its own taste, and that tends to get bound up with the other things settling out which can make for some confusing flavors early on. For example, I find that the first few pints of beers with roasted grains will taste way roastier than the rest. I assume that's either roast-coated yeast or fine bits of husk material from the roasted grains settling out. Same with hops.
Alestate - I completely agree that clarity and maturity are not the same thing. Finings get me partway there, but some of the maturation seems to only happen with time (or some magic combination of finings that I haven't tried yet). Of course if we had in-home centrifuges we could probably improve on the finings just a bit....