Is this conception of countable vs. uncountable infinity adequate? I am not a mathematician, so please point out any mistakes I am making here - I am trying to grasp the concept of countable vs. uncountable infinity in a somewhat informal way and would like to know whether that conception makes sense.  

We can imagine the set of natural numbers as an axis that goes from some fixed point ($0$, or $1$ if you want) to infinity:


Clearly, the points on this axis are countable, because we know exactly how the axis goes on and can therefore make a precise calculation about how many points the rest of the axis will contain.  

For the integers, we no longer have a fixed starting point, but the axis grows infinitely in two directions:


However, we still have only one axis of fixed points and can make a precise calculation of how many points the axis as a whole will contain.
The first thing that bugs me: Since the axis is calculably exactly twice as long, this should be a "larger infinity" than the for the natural numbers, right? But still, we would say that $\mathbb{Z}$ has the same cardinality as $\mathbb{N}$?

For the rational numbers, things get a little more difficult, but we can still handle it: Any rational number can be displayed as the fraction between two integers - if I understood it correctly, this is what the Cantor pairing function does? - so we can just add a second axis to account for the combinatoric possibilities yielding $\mathbb{Q}$:


The amount of points now doesn't simply add up, i.e. the axis doesn't just get longer (as in the step from $\mathbb{N}$ to $\mathbb{Z}$), but it multiplies, i.e. there are more axes now, so that's even a "larger increase of infinity". Is this correct?
But we still have finitely many axes with countably many points, so the whole amount of points is countable too.  

Now for the case of real numbers, things look a bit differently.
Clearly, a one-dimensional system doesn't suffice because we need to account for the digits behind the comma, so we need at least two axes, in order to create $0.0, 0.1, 0.2, ..., 1.0, 1.1, ...$:

Now that doesn't suffice either, because from $1.1$, we can decide to either stay at $1.1$, which would be $1.10$ (Is it true that $1.1$ is in fact $1.10$ which is in fact $1.1000000...$, so that rational numbers are actually never really finite, or is this idea false and $1.1$ is really just $1.1$?) or go further to $1.11$, so we need another axis:

We are now three-dimensional and can thus account for all the numbers with two digits behind the comma, but that still doesn't suffice, because between $1.10$ and $1.11$, we also have the numbers $1.101, 1.102, 1.103, ... $, and from any point we are, we are recursively stepping one dimension deeper, because for any digit we add, we again have all the possibilities to go on from that point, so we never reach a point where we can stop adding axes:

(At this point I'm running out of imagination on how to draw a 7D diagram, sorry)  

Now we are at the point where we run into an infinite number of axes - and this corresponds to the set of real numbers $\mathbb{R}$ no longer being countably infinite.  

My question is:
Is it adequate to say that countably infinite corresponds to finitely many dimensions, while uncountably infinite corresponds to infinitely many dimensions, or did I go anywhere wrong in my conception?
The notion of "countably infinite" is well named.  Another word you can use is "enumerable," which is even more descriptive in my opinion.

I understand your intuition on the subject and I see where you're coming from.  Let me try to give you some insight into the agreements about infinity that have been reached over the years by the mathematicians who've worked on this problem.  (This is what I wish someone had explained to me.)

"Countably infinite" just means that you can define a sequence (an order) in which the elements can be listed.  (Such that every element is listed exactly once.)

The natural numbers are the most naturally "countable"—they're even called "counting numbers"—because they are the most basic, natural sequence.  The word "sequence" itself is wrapped up in what we mean by natural numbers, which is just one thing following another, and the next one coming after that, and the next one after that, and so on in sequence.

But the integers are countable as well.  In other words, they are enumerable.  You can specify an order which lists every integer exactly once and doesn't miss any of them.  (Actually it doesn't matter, for the meaning of "countable," if a particular element gets listed more than once, because you could always just skip it any time it appears after the first time.)

Here is an example of how you can enumerate (count, list out) every single integer without missing any:

$0, 1, -1, 2, -2, 3, -3...$

The rational numbers are also countable, again because you can define a sequence which lists every rational number.  The fact that they are listed means (at the same time) that they are listed in a sequence, which means that you can assign a counting number to each one.

The real numbers are not countable.  This is because it is impossible to define a list or method or sequence that will list every single real number.  It's not just difficult; it's actually impossible.  See "Cantor's diagonal argument."

This will hopefully give you a solid starting point to understanding anything else about infinite sets which you care to examine.  :)
Your understanding of the whole thing is a little off the road of regular Mathematical sense. I am not a mathematician or even a math major, but I know some math and let me tell you some of my understanding. The major difference btw set R and N is that N is listable but R is not, and listable is just another saying of countable. N is somewhat discrete while R is "continous". Btw every 2 reals, there will always be a real, no matter rational or irrational while between 2 consecutive integer, no integer exists. There is no "next" for a real. As to the dimension thing, you have to know linear combination and dimension is defined as the minimal number of elements such that each element of the space can be represented as a linear combination of those elements. Those elements must also be linear independent. In finite dimensional space, the situation is always simpler than infinite dim space. In finite dim, people tend to find element whose span is dense instead of being the whole space. For R, it is one dimensional because a single real can represent all others by linear combination.