1. Function composition middleware
Actually, I simply use classes as a way to do function composition. The reason I chose classes over functions I think doesn't really matter in the grand scheme of things, but in my talk I mention that what I am doing is function composition, and I spent a solid 5 to 10 minutes explaining how function composition solves the problem. :) We're on the same page here.
2. The fact you don't have the requirements for the situation I posted.
I understand this, and that is why Vagrant works for you. But it also works for people who do have requirements other than yours. There is a _huge_ amount of Windows users out there that love Vagrant because it now makes Ruby on Rails and so on development work well! If I catered Vagrant to only _my_ requirements (Mac OS X host, Ubuntu guest), then Vagrant would indeed be a lot simpler, but Vagrant as a tool gets a lot of its power and usefulness out of its ability to work in heterogenous environments.
3. `vagrant ssh` is slow. This must be caused by complexity.
This has nothing to do with complexity. This is caused by poorly optimized code paths and how things work up to that point. I just want to note that nowadays `vagrant` commands across the board are a LOT faster. But there is still a lot of work to be done. Specifically the `vagrant ssh` case will get a massive speedup due to cached SSH info.
I specifically have issue with "complexity causes slowness." Sure, it happens, but it doesn't HAVE to, and I'll show that to be true.
4. Plumping and porcelain Git vs Vagrant
Perhaps there is an option here to use unix-type piping to provide an API via plumbing commands. I've decided to take a different approach. Will this work well? Time will tell, but I don't think its fair to criticize Vagrant at this point before this is proven/disproven yet.
5. 12000 lines of Ruby.
~3500 is test code.
~5500 is "plugins" but there is a lot of boilerplate here. The boilerplate is on purpose, I didn't want any "magic," hence it being somewhat Java-like, I suppose, but this is a lot more friendly to non-Ruby developers, and a vast majority of Vagrant users are not Ruby developers. Note that ~3000 of this is ONLY the VirtualBox plugin, which is a pretty complex piece, so that makes sense. The other ~2500 is _every other_ feature of Vagrant.
~3300 is core code. This is mostly a glorified plugin runner that handles plumbing for you, such as hooking up the right host/guest combination, choosing the right communication (SSH, WinRM, etc.) for the machine, error handling, etc.