Things possible in IntelliJ that aren't possible in Eclipse?

CTRL-click works anywhere

CTRL-click that brings you to where clicked object is defined works everywhere - not only in Java classes and variables in Java code, but in Spring configuration (you can click on class name, or property, or bean name), in Hibernate (you can click on property name or class, or included resource), you can navigate within one click from Java class to where it is used as Spring or Hibernate bean; clicking on included JSP or JSTL tag also works, ctrl-click on JavaScript variable or function brings you to the place it is defined or shows a menu if there are more than one place, including other .js files and JS code in HTML or JSP files.

Autocomplete for many languagues

Hibernate

Autocomplete in HSQL expressions, in Hibernate configuration (including class, property and DB column names), in Spring configuration

<property name="propName" ref="<hit CTRL-SPACE>"

and it will show you list of those beans which you can inject into that property.

Java

Very smart autocomplete in Java code:

interface Person {    String getName();    String getAddress();    int getAge();}//---Person p;String name = p.<CTRL-SHIFT-SPACE>

and it shows you ONLY getName(), getAddress() and toString() (only they are compatible by type) and getName() is first in the list because it has more relevant name. Latest version 8 which is still in EAP has even more smart autocomplete.

interface Country{}interface Address {    String getStreetAddress();    String getZipCode();    Country getCountry();}interface Person {    String getName();    Address getAddress();    int getAge();}//--- Person p;Country c = p.<CTRL-SHIFT-SPACE>

and it will silently autocomplete it to

Country c = p.getAddress().getCountry();

Javascript

Smart autocomplete in JavaScript.

function Person(name,address) {    this.getName = function() { return name };    this.getAddress = function() { return address };}Person.prototype.hello = function() {    return "I'm " + this.getName() + " from " + this.get<CTRL-SPACE>;}

and it shows ONLY getName() and getAddress(), no matter how may get* methods you have in other JS objects in your project, and ctrl-click on this.getName() brings you to where this one is defined, even if there are some other getName() functions in your project.

HTML

Did I mention autocomplete and ctrl-clicking in paths to files, like <script src="", <img src="", etc?

Autocomplete in HTML tag attributes. Autocomplete in style attribute of HTML tags, both attribute names and values. Autocomplete in class attributes as well.
Type <div class="<CTRL-SPACE> and it will show you list of CSS classes defined in your project. Pick one, ctrl-click on it and you will be redirected to where it is defined.

Easy own language higlighting

Latest version has language injection, so you can declare that you custom JSTL tag usually contains JavaScript and it will highlight JavaScript inside it.

<ui:obfuscateJavaScript>function something(){...}</ui:obfuscateJavaScript>

Indexed search across all project.

You can use Find Usages of any Java class or method and it will find where it is used including not only Java classes but Hibernate, Spring, JSP and other places. Rename Method refactoring renames method not only in Java classes but anywhere including comments (it can not be sure if string in comments is really method name so it will ask). And it will find only your method even if there are methods of another class with same name.Good source control integration (does SVN support changelists? IDEA support them for every source control), ability to create a patch with your changes so you can send your changes to other team member without committing them.

Improved debugger

When I look at HashMap in debugger's watch window, I see logical view - keys and values, last time I did it in Eclipse it was showing entries with hash and next fields - I'm not really debugging HashMap, I just want to look at it contents.

Spring & Hibernate configuration validation

It validates Spring and Hibernate configuration right when you edit it, so I do not need to restart server to know that I misspelled class name, or added constructor parameter so my Spring cfg is invalid.

Last time I tried, I could not run Eclipse on Windows XP x64.

and it will suggest you person.name or person.address.Ctrl-click on person.name and it will navigate you to getName() method of Person class.

Type Pattern.compile(""); put \\ there, hit CTRL-SPACE and see helpful hint about what you can put into your regular expression. You can also use language injection here - define your own method that takes string parameter, declare in IntelliLang options dialog that your parameter is regular expression - and it will give you autocomplete there as well. Needless to say it highlights incorrect regular expressions.

Other features

There are few features which I'm not sure are present in Eclipse or not. But at least each member of our team who uses Eclipse, also uses some merging tool to merge local changes with changes from source control, usually WinMerge. I never need it - merging in IDEA is enough for me. By 3 clicks I can see list of file versions in source control, by 3 more clicks I can compare previous versions, or previous and current one and possibly merge.

It allows to to specify that I need all .jars inside WEB-INF\lib folder, without picking each file separately, so when someone commits new .jar into that folder it picks it up automatically.

Mentioned above is probably 10% of what it does. I do not use Maven, Flex, Swing, EJB and a lot of other stuff, so I can not tell how it helps with them. But it does.

There is only one reason I use intellij and not eclipse: Usability

Whether it is debugging, refactoring, auto-completion.. Intellij is much easier to use with consistent key bindings, options available where you look for them etc. Feature-wise, it will be tough for intellij to catch up with Eclipse, as the latter has much more plugins available that intellij, and is easily extensible.

Probably is not a matter of what can/can't be done, but how.

For instance both have editor surrounded with dock panels for project, classpath, output, structure etc. But in Idea when I start to type all these collapse automatically let me focus on the code it self; In eclipse all these panels keep open leaving my editor area very reduced, about 1/5 of the total viewable area. So I have to grab the mouse and click to minimize in those panels. Doing this all day long is a very frustrating experience in eclipse.

The exact opposite thing happens with the view output window. In Idea running a program brings the output window/panel to see the output of the program even if it was perviously minimized. In eclipse I have to grab my mouse again and look for the output tab and click it to view my program output, because the output window/panel is just another one, like all the rest of the windows, but in Idea it is treated in a special way: "If the user want to run his program, is very likely he wants to see the output of that program!" It seems so natural when I write it, but eclipse fails in this basic user interface concept.

Probably there's a shortcut for this in eclipse ( autohide output window while editing and autoshow it when running the program ) , but as some other tens of features the shortcut must be hunted in forums, online help etc while in Idea is a little bit more "natural".

This can be repeated for almost all the features both have, autocomplete, word wrap, quick documentation view, everything. I think the user experience is far more pleasant in Idea than in eclipse. Then the motto comes true "Develop with pleasure"

Eclipse handles faster larger projects ( +300 jars and +4000 classes ) and I think IntelliJ Idea 8 is working on this.

All this of course is subjective. How can we measure user experience?