What is the key for a successful web site?

I'll answer from my point of view so maybe my ideas aren't perfect you need :
1- Security : you don't need attacker destroyers your web site after give huge efforts on it.

2- Friendly user interface : a kindly web site is much better, check all the major web sites in the world give huge efforts to make the UI as easy as it could be.

3- No Errors : you need to check for functional error , JS errors .

4- Validations
5- SEO (Search Engine Optimization ) to increase site traffic

the best tow words for this :- usability and Specialization

In reality, it can seem like an endless list but if you look carefully you’ll see that there are certain elements that are more important than others.
1.
Good Domain Name easy to remembered 2.
Professional Design:the first impression for website visitor will decide whether your business is worth their time.
So its need to be (Easy to navigate,Must display correctly and quickly on all browsers,Capture attention) 3.Search Engine Optimization (SEO): is the process of positioning your website to be found on these search engines which increases the traffic to your website.
4.
Content (Useful, informative, free, and easy to understand) 5.Know your Competitors and try to be the best.
6.Use Social Media to build an interested online community.
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What you need to have success web site depend on what the site build for and for whom then you can decide how you can build it in best way

The key is ignore as much as possible (apart from security of course), make it simple make it easy to handle, don't put to much information and features on it.
Stick to a concept but don't mix to much like flash, videos, sounds colors.
Some pages are overloaded with (sorry) "crap" and it's nearly impossible to find what you were looking for.
It's a fact that the human mind can process 7-9 elements on a site at once, everything above that just gets secondary or even lost, except you have some really annoying flash animation at the corners, and tbh.
that always appears cheap to me.
A site should be self explaning, it should please the eyes and it should make you courious to stay and browse it.
My favourite example of the most successfull website will allways be Google.
A logo, a textbox and a search button.
The one and only requirements you ever need for a seach engine.

Agree with Zaid and addition with him: - Usability: You site navigation should be very easy to use so visitor shouldn't need to search the option to perform actions.
- Interesting Content: Every site builds on different idea so website content varies website to website.
But your content should be fulfilling all requirement of your website and it should be interesting for your site visitor.

Simplicity Usability User friendly Richness of content Good Performance Credibility which you earn by time

A good Knowledge about SEO

Most of the important aspects of a good website(usability, security, design, SEO etc) have already been discussed above.
Some of the other things that should be taken into consideration are 1) Cross browser optimization - Ensure that your website appears properly across various browsers 2) Optimization for mobile browsers as well - Today most of the people access the web through smart phones.So ensure that your page does not get distorted on smaller screens 3) Web optimized images - preferably 72 dpi instead of the usual 300 dpi.
lack of good images can impact the traffic to your website.
4) Credibility - A good website instills trust from visitors.
Good website design takes this into consideration by incorporating items that help identify that you are a legitimate resource.
5) Content - Content is king, however, too much content, or stale or bland content will destroy your website.
In addition to meaningful content, posting frequency is also equally important 6) Simplicity and even using the right kind of font can make a difference 7) Apart from SEO, make use of other important analytics software to get important statistics for your website to track if the website is meeting its requirement 8) Last but not the least, a good "About Us", "Contact US" & "FAQ" page.

1.
An attractive interface 2.
Useful information 3.
Easy access to all the links in the website 4.
Special services 5.
Advertising of course

well definitely a site should execute it's core functionality from start till end without any errors.
so core functionality should work fine.
then second most important thing is ease of use the biggest example of this is Facebook VS Orkut, there was ease of use that made people to move to FB

A good and successful website should 1.
Have a Professional look and feel, with easy and understandable user interface to a lay man 2.
It should be cross browser, optimized and should be adaptable to the screen resolution of all end users.
3.It should be globally web standard compliant and better if browsers not need to install special plugins separately but must implicitly support browser.
4.It should be developed using latest and better performance oriented frameworks 5.
Faster request processing is key for good website 6.Multilingual support 7.The website should be hosted on globally connected high capacity servers to avoid data loss and easy backup.
8.Should be accessible to social web like facebook, twitter, etc, etc.
9.Search Engine Optimized.
10.
High Security, firewalled from malicious access ofcourse.