Saving a defense set up

It would be nice to set up your defenses for a map, and then be able to save that set up, so every time you start a map you do not have to redo your defenses. 

But some maps you cant build all your defences in the first wave as there isnt enough mana :)

The joy of playing is the build! i love being the builder and trying to build as fast as i can - then trying to beat that time :D 

Correct, a lot of maps, such as Forest Biome, do not give enough mana at the start for a full build.  Just saving the first wave build is good enough though.  

Also I thoroughly enjoy building too!  This is why I think saving a build would be so great.  You can see exactly how you had laid it out last time, and make fine tune adjustments to your build until you can get a perfect set up.  Also you can then compare your last build to a new build as well.  Often times I find myself forgetting exactly how I laid out my proton beam/fire aura/trap placement and I feel being able to save the build will actually make the building more fun, and also lead to more intuitive builds.

There are quite a few times where very small micro adjustments in placement can mean the difference (especially for a solo player) between a berserker making it to one of your barricades and crushing it, or the berserker being killed. 

A build that has been saved will definitely add to more players thinking about how they are building the map, and tweaking it.  However, it will also aid players in being able to set up and try crazy new builds as well.

The last thing it affects is farming speed.  It will greatly enhance farming speed, especially when you need to farm for that last chaos shard you need.

But what if the build consisted of towers that were on a toon you don't have equipped?

I see your point but the game does keep all the towers at the end of a map now and allow you to run around.

Previously DD would sell all your towers and you didn't know where you had built!

Things such as towers not available to current hero deck can just result in the towers not being built.  A red text can be displayed stating what towers did not get built, and then you can facepalm and say oh ya, I forgot to place that hero in my hero deck.