.... (And the hardgrapple biteybears made me burst out laughing.) ...
Ditto.
I agree that the flora and fauna can give a feel of the lands one travels through, and that they should definitely be used to convey a sense of such. The noting of occasional goings on is a good thing, as is the basic description of the terrain - of what ever form it is.
The impact of their descriptive use I think really depends on the party though. If the characters are involved at such a level that they want to know where the oak grows for their furniture making business, or where to trap the fur bearing critters for their hat business, then it takes on a lot more importance. For those that only care about making their survival check as they pass through because they didn't stock up on traveling rations, it matters little to them.
Weather is another huge element that seems to be overlooked too. And tying into weather, calendars. But then, like so much else, if the DM and party don't care, it's of little use. But for those that do care, it can shape the campaign timeline, and even the events within them. In my last campaign, the party spent their careers far north. When winter came, it was cold and harsh - lots of snow and freezing temps. Effectively, any wars or trade halted during the winter months due to the inability to move freely (or at all in some cases). Even the PCs planned their adventuring season such that they would be "home" for the winter. The "world" didn't start moving again until spring when everything started to thaw.
Adding in a mix of critters and weather and all the bits that tie in, and you can have a very active and immersive world - and the DM doesn't even have to be terribly in depth about it. A note at the begining of the session, or during changes, and that's normally enough. (Unless of course environmental Fortitude checks need to be made.) More detail can always be added as the players ask.
Anway, just my two shekels. (Yeah, sorry. Got kinda rambly there...)