Q:

Why there is no tag cloud to choose tags from?

I feel like hunt and peck on the Stack Exchange network when choosing the tags for a question. How one should know which tags exist when there is no list or cloud of tags to choose from?

A:

Piotr Migdal has created a nifty piece of software that creates "tag maps" for Stack Exchange sites.
The original was created for Cross Validated, and they also been created for other sites, including Biology and Chemistry.
Presumably as a stress test, he has also created one for Stack Overflow—but only including the top 256 tags (only ~0.6% of the total!). It's still rather insane: 
  
Yes, it's very cool to look at. It's also interesting for the study of trends. However, I don't think it's particularly useful as a way of selecting tags. At least not until the Stack Exchange folks start rewarding everyone who hits the rep cap with a 30" monitor.

A:

Firstly, there are far too many tags in Stack Overflow for a tag cloud to be anywhere near usable.
Secondly, tag clouds are not actually used much in the real world anyway. They're a fad that has more or less died out (thankfully).
As Jakob Nielsen wrote back in 2009 about Tag Clouds

...usability studies show that most normal users don't know what they are and don't know how to deal with them.

and 

Just because something looks cool, and just because it's a current fad in Web design, don't mean that it's necessarily best for usability

Finally, there is the AutoComplete tag entry system when adding questions, which does work pretty well for finding tags. Much better than an oldfashioned Tag Cloud.

