First of all, Xcode comes with gcc, gdb, make, and all the other usual Unix developer tools. And of course PHP, Python, Perl, Ruby, Java (and others?)
But for GUI applications Cocoa is an excellent API built on a great language, Objective-C. Objective-C is a thin layer on top of C that adds very dynamic object oriented features (actually somewhat similar to Ruby). It might take a little while to get used to the syntax, but it's actually quite readable and pleasant.
Cocoa is pretty verbose, but between the named parameters (sort of) and verbose API names, it ends up being very readable code, almost like you're reading sentences of English. Some people don't like it, especially at first.
Apple has lots of documentation for iPhone devs too, which is also useful for new OS X devs. In particular:
* The video entitled "iPhone Application Development - Getting Started" @ http://developer.apple.com/iphone/index.action
* http://developer.apple.com/iphone/gettingstarted/docs/object...
More detail on Objective-C: http://developer.apple.com/documentation/Cocoa/Conceptual/Ob...
And Cocoa: http://developer.apple.com/referencelibrary/GettingStarted/G...
The nice thing about Xcode is that when you don't need or otherwise want to avoid the GUI integration, you can: build rules can run anything you want, like a makefile or other script, and even show you the raw output in a terminal-like split view. Apple provides command-line access (/usr/bin/xcodebuild) for those cases where you want to start with an IDE-style project but still have direct build control from outside scripts. I've only dabbled in VS2005, but Microsoft's environment never seemed "avoidable" in the same way. The GUI tangled with every aspect of the project. It was so awkward that I once cranked open "vim" from Linux (over SSH) to edit raw VS2005 XML, and it was much faster than tweaking the project through the GUI!
Speaking of "vim", I often use this for editing on Mac OS X. It depends on the type of edit, I suppose; Xcode has some strengths (e.g. pulling a whole function out is easier with the mouse, and I often do edits based on project search results). But "vim" is deceptively simple, and is ideal for many kinds of edits. It has some pretty addictive features, like simple paren-matching with "%" even when in the middle of a function name.
It's really nice that Mac OS X ships with many scripting languages, because your project can depend on them. For instance, you can automate many tasks in Python and Perl, and other Mac developers can still build your code without a headache. Windows sucks for this; if you share a bunch of code that's heavily automated with scripts, odds are people will have to download and build all the interpreters you're using (and of course, you probably need a batch file to run it, because #!/usr/bin/perl is meaningless to Windows). It's just a completely different environment...I find Mac development infinitely more productive for many reasons but the Unix tool set alone is an unbelievable starting point.
The long answer is that unless you're porting an existing C++ app, you'll want to learn Cocoa if you want to write GUI apps. It's both the only supported 64-bit API, and what the iPhone uses. There are bindings for Ruby and Python, but the native language is Objective-C. There's also a semi-supported Scheme-like language called [Nu](http://programming.nu/).
The [Purple Newfie](http://books.google.com/books?id=ns6tB-uXuxcC&dq=learnin...) book is pretty good for going the Objective-C route, but quite dated at this point.
Once you've worked through the book, you might play around with Core Data (http://developer.apple.com/documentation/Cocoa/Conceptual/Co...) and Cocoa Bindings (http://developer.apple.com/documentation/Cocoa/Conceptual/Co...), which the book totally doesn't cover but are very cool for rapid development.
And if you're deranged like me you might enjoy just randomly playing around with Interface Builder first to get a feel for what's possible without too much coding.
Mac is very similar to Linux in the way most users approach the dev. environment. That is: souped up editors in favour of full IDE's. That being said XCode (from apple) would be the VS of the mac world for cocoa apps. Of course Eclipse is present as well.
Most popular and "macish" though is text-mate. That's what all the cool kids use.
Having moved from windows previously, I can tell you that the level of integration and features from VS and the like is not found on either Mac or Linux. That will take some getting used to, but I've found that it's not too much of a big deal once you get over the initial shock.
If you're using VisualStudio on Windows, XCode is what you'll want on Mac. The other comments here about Emacs and whatnot aren't really relevant because if you were a die-hard Emacs / CLI guy you'd be using them on Windows too.
I don't really use .Net or VS but yes, I agree that he should definitely take a look at Xcode.
If you're developing web applications or anything portable then use whatever you like. You can find for Mac versions of the standards: Emacs -> Aquamacs, Vim -> MacVim, etc.
There are some Mac-only development tools in various categories so you might want to check out sites like MacUpdate, Macworld, MacMerc. For example, if you pair program with others, definitely check out SubEthaEdit.
It's kinda weird, I know, but I just prefer to use Emacs for editing instead of Xcode.
depending on what you do though that may not be the answer. lots of ways to develop for mac. is there something in particular you had in mind? xcode is the absolute wrong answer for some things (like java, ducks flames).
For web apps being written on a Mac, TextMate.