*Headache* Yes, Nirn is pretty limited to Earth's geography, because that's how it's always been. Unless they wanted to retcon that too. Spherical planets orbiting a sun, leading to seasons and bound to geographical climates. Flora such as Chorrol's Great Oak, the Silver Birch north of Chorrol towards Bruma, the Scots Pine and Redwood on the path to Skingrad. Blue skies indicating the presence of oxygen and nitrogen atoms scattering the sun's rays.
I played an online game called Aion for a while. It had deserts right next to snow covered plains and mountains. I quit pretty soon after that.
The magic of TES games is the immersion it provides. It has earth-like rules because that's what we're used to, as do all fantasy stories more or less. Making up a whole new set of rules, and whole new species to play as or read about would be difficult to follow and therefore difficult to become immersed in. So you find a happy medium, where the basic rules we understand are catered for, and a healthy dose of epic fantasy laid on top of those groundworks. If Skyrim had all those habitats that you just listed... that breaks the rules and breaks the immersion. Unless an explanation is provided as to why a desert is found next to snow plains in Skyrim (work of a god, place of magic, being common examples) then it can't work and still be acceptable.
"We included more landscape variety because people complained about Oblivion's monoculture" is just taking the problem to the other extreme end of the scale. I'm sure Bethesda will find the perfect balance. They're a great development studio.
*Headache* I don't think you actually understand me and I really cba to take the time to fully explain why. In short though, I don't think our opinions are actually at odds... I'm not saying I want Beth to put a desert next to a snow bank. I'm not saying I want a lush jungle oasis on top of what should be a deserted arid mountaintop. I'm not saying I want a green sky, or new laws of physics, or life forms based on something other than carbon... Or for that matter any form of plant/animal life that doesn't make sense in the environment in which it is placed.
What I'm saying is that I want to see a more vivid, unique, and varied game world. You can see vivid/unique/varied terrain and life all over Earth, much more so than in Oblivion. So I don't see how my request is invalid even if the game has to restrict itself to earthly influences. And personally I don't think it should. Morrowind had many fantastic elements to its world that you would never see on Earth, such as certain architecture, silt striders, and various plant and animal life. Why is it unreasonable to ask for similarly original and fantastic environments in the next ES game? Why is it unreasonable to ask that in an entire province, roughly 15% of the landmass of an entire CONTINENT, that there be a wide variety of terrain/landscape types?
I live in California and in this single state we have a very wide variety of terrain, including desert, forest, mountains, hills, valleys, bays, beaches, lakes, volcanoes, mesas, etc. California's land mass should theoretically be roughly the same as any province in Nirn, if not significantly smaller (California takes up far less space on a map of North America than Skyrim does on a map of Tamriel, though I'm not sure how large Tamriel is supposed to be relative to North America)... We also have thousands of species of wildlife, including hundreds of types of birds, small game, large game, livestock, domesticated animal, predators, sea creatures, and more. And thousands of types of plant as well... In fact, of the almost 6,000 types of plants native to California (i.e. that grew before it was settled), about 40% are unique to the state. Technical limitations aside, why shouldn't a province of equal or greater landmass in a FANTASY GAME display at least as much variety in its flora, fauna, and geography? Why shouldn't we expect to see new plant/animal types or new forms of terrain?
People have already demonstrated that stereotypes regarding northern lands on Earth are not as sweeping as many would believe; northern European countries are not 100% ice/snow; parts of Canada may not see snow all year, and most of that country is usually not snowy... And even those lands that are mostly snow still feature a wide variety of terrain and wildlife; they're not just endless mountains, glaciers, and tundra... So again why is it wrong to ask for a landscape that contains vivid and varied geography, plant life, and animal life? Even if they make Skyrim a virtual clone of a northern European country -- which IMO would be a huge mistake and a big letdown -- they could still make the geography much more interesting than Oblivion's geography was.
You talk about immersion. To me, a large world is more immersive if its terrain and wildlife are diverse and vivid. Oblivion's landscapes were anything but. Anywhere in the world you could find the same rocks, same trees, same deer/bear/mountain lions... Each town had relatively unique architecture but otherwise there was hardly any variety in the game world, and that made it far less immersive to me.
Also, final note: again using California as a real life example... You say it makes no sense to put a desert next to snow plains, and while in principle I agree, I should point out a couple things: firstly, many deserts actually receive a lot of snow every year -- a few years ago (2004), deserts in Southern California (which often reach 120 degrees Fahrenheit in Summer) were covered in snow by storms... This is in some of the hottest and driest deserts in the US btw. And secondly, Death Valley, one of the lowest, driest, and hottest areas in the continental US, is situated within about 75 miles of the tallest peak in the continental US, Mount Whitney. So it's not impossible for a low, dry, arid desert to be situated next to a towering peak so tall that its peak rests decidedly above the tree line.
Hm, this post ended up being a lot longer than I originally intended. Ah well...
tl;dr: I don't disagree with you. I believe that the game world can be made far more diverse including a wide variety of flora, fauna, and terrain, while still maintaining reasonable "realism" with regard to ecosystems you could expect to see on Earth or an Earth-like planet.