This is the full interview for this FastCo article

1. What is so irritating about losing this functionality? Is using
double quotes THAT much worse? 

People get irritated when they are forced into changing a comfortable habit.  

2. What do you think Google has to gain (if anything) by deleting this

Maybe just cleaning up the interface.  The + is a hidden feature (in the sense that it's impossible to discover unless you read the docs or someone tells you), and it comes from a time where the search query was considered a "command", to be interpreted strictly according to Google's "query language".  The search query has been moving more towards a more "natural" interface, where you type what's on your mind and Google infers what you mean -- more like talking to a person than issuing a command to a computer.  When I'm writing to you, I don't put a "+" in front of the words I want to you to interpret literally.  Why should I do so when writing to Google?

But that's kind of a design-perspective reason, and it's probably not the actual reason in this case.  Unless Google is removing other operators and hidden features, you're probably right about the Google+ thing.

3. An argument I'm going to make is that UIs should ideally be "deep"
-- that is, simple, intuitive and error-tolerant on the surface, but
with levels of more "surgical" functionality built-in below for users
to discover and exploit as they grow more skilled with the software.

A lot of designers would say they strive for that, yes.  But I don't like that as an ideal at all.  For me, the ideal UI offers many uses for many people, but accomplishes that without hidden "deep" features.  A piece of paper, for example, can be used for all sorts of things -- pictures, poetry, airplanes, spitballs, stabilizing a wobbly table -- but it doesn't offer those as "hidden features".  It's just flexible.

Here's an example relevant to the Google problem.  In the new version of Mail on the Mac, when you type a search term, it offers a menu, where you can clarify what you mean:

If I clarify that I'm referring to you (which I can do just by hitting the down arrow key), the term turns into a token:

And I can more tokens, similarly:

This allows me to build up very sophisticated queries.  But there's no hidden feature here -- nothing to "discover".  There's nothing reserved for "power users".  It just offers wonderful flexibility with what I type.

You can imagine a similar sort of thing in lieu of Google's + operator.  When I'm typing a word, it suggests that a drop-down is available:

And if I choose that, it offers some options:

This is off the top of my head, so it could be a lot better, but you get the idea.  The point is that Google currently treats the query field as plain text, like a command line, so it inherits that sort of "hidden command culture".  If you take a more flexible view of text -- if you think about how text itself can offer rich interaction -- then you no longer need the hidden commands, and the "power features" are available for everyone.