Every age is a different dimension, with different landscape, blocks, maps, etc.
Ages can be differentiated by a large number of "artistic" factors, such as colorations (fog, sky, etc), number of celestial objects (sun, moon, stars fields), their speeds and directions, etc. These have no actual effect on the game, except to the extent that suns provide light when overhead.
Ages can be differentiated by many things that do affect the game as well. Whether or not your age has Lakes, Mineshafts, Caves, etc.; all the features of vanilla worlds can be present or not.
There are also many new features that can be found. Tendrils can provide streams of materials, typically wood, as a form of "world root". Spheres can be found both in the sky and in the ground, made of a variety of materials.
Floating islands can even provide "biomes in the sky", while plunging the world underneath them into darkness.
Skylands can carve away very large amounts of landscape, leaving you with isolated little bits of land, which will rarely or never go down to the lava level.
Caves in ages will generally NOT have lava when down deep; Xcw has explained that at the time the caves are made, it is very hard to tell if there was land there and if so, replace that with lava. (To clarify: there is **no lava below Y=11** in the current 0.10.12.1 (still true in 0.11.0), and it isn't clear that there will be anytime soon. If you think you can solve this problem, then try to decide how caves and ravines can determine if the blocks that are being replaced were blocks from natural ground creation, or if it was just a stone sphere lying in a void or something else that is other than natural ground).
Worlds that look similar to the overworld are fairly easy to create. Exact matches can be made with "native biome controller" (see Writing overworld clones), but the normal mystcraft biome layouts ("normal biome distribution", and the "tiny" - "huge" variations) will not match the overworld -- neither vanilla style rivers, nor the "special biomes" (beaches, shores, edges; interior hills) will happen the way the overworld works. Rivers, in particular, will look like shallow oceans, without any islands. Note that using "native biome controller" is subject to all the restrictions put on the overworld -- biomes that are set to only show up in ages will not be seen in such a world. The use of NBC with mods that alter overworld generation -- Better world gen, or ATG -- has not been tested. The use of Climate Control 0.3 and Mystcraft 0.10.15.0 in 1.7.2 will cause a crash on NBC worlds (this is reportedly fixed in 0.11.0 with the newest CC).
Worlds that look like the nether, however, are not easy to create. Worlds that are close to the nether can be written -- there is a Cave World layout that mimics the nether, and things like Nether Fortresses and Glowstone Crystalline Formations can also be found. However, the ground layout of gravel, soul sand, etc, found in the nether is not directly duplicatable at this time. Additionally, the nether has a natural "dim lighting" -- it is not completely dark even though there is no sun. Mystcraft has Bright lighting, but it does not have a nether-compatible background lighting.
Not all worlds have the same type of ocean. Island worlds, for example, will have no ocean by default; almost any mod-added liquid, or ice, can be used as the ocean in an age.
Instability is a serious concern. Naturally occurring "unusual" worlds, as well as manually written inconsistent worlds, will have some form of "instability". This can range from a hostile surface (generally potion effects, or entities catching on fire), to potion effects that happen underground, to spontaneous explosions, to the world falling apart -- either crumbling to sand (or a lesser form of erosion around lakes), to Decay spreading, to Meteors raining down from the sky. The most common "unusual" worlds will be worlds where a liquid other than water, ice, or lava is used as a lake or ocean; or where extra ore deposits are found, either as small occasional lumps, or large, long tendrils -- a "cave-like" system filled with an ore instead of with air.
Decay can range from simple colored blocks that spread, replacing the ground (and ores) with copies of themselves; to Black decay, which will cause sinkholes to the void; to White decay, which will harm any entity that moves on it (and will spread while you stand still to heal).
Finally, and critically important, Entrapment is a serious risk to the age traveler. Both the high-end of Decay, and Meteors, can destroy the world if the world is badly written or full of contradictory rules. Even a stable world can have openings to the void -- both Skylands (which is a terrain alteration that can affect any world type), and Island World (which is a world type) will result in no bedrock layer, and nothing to stop you from falling out of the world, taking everything you are carrying with you. Spontaneous Explosions and Crumble will ultimately destroy any chests, bookstands, or other stationary exit you try to set up. All travelers are encouraged, very strongly, to always carry a way home with them, and use it if you fall into the void or are being crushed by falling rock.