There are a number of ways guaranteed to turn-off visitors to your web site.
These are worth considering as you either design a site, or revise a site.
Turn-offs generally can be defined as anything that impedes the user from quick and easy (intuitive) use of your site.
Turn-offs also are often the result of gratuitous use of technical 'bells and whistles'.
One of the major turn-offs is a sloooow loading site.
There can be a number of contributing factors to this:

Large graphics files (meaning the file size in bytes).
Graphics files need to be optimised to reduce the file size to a minimum to enable them to travel through the Internet in minimum time.
Most graphics software enables you to do this.

Splash Screens.
These are often Flash graphics and can take a long time to 'load', especially where there is limited bandwidth.
Bandwidth is the carry capacity of the Internet, the bigger the faster - a bit like comparing a fire hose and a garden hose.
Splash screens also annoy repeat visitors, as they revisit the site and have to wait for the screen to load (not everyone knows how to properly Bookmark).

Excessively long pages.
Pages should generally be no longer than three screen-fulls (scrolls).
Also, bear in mind that most people scan web pages rather that read in detail (until they find the specific page they seek).
So, unless the page is a scholarly article, make use of short sentences and/or bullet points.
Other turn-offs include:

Background music that automatically starts as the page loads.
As well as slooowing the page loading down, your choice of music is unlikely to please everyone.

Set a screen resolution too wide for the majority of computers in use.
You would be surprised how many computers with 800 pixel x 600 pixel screens are still in use.
So, set Table widths as a percentage of screen width, otherwise many of your visitors will be annoyed by needing to utilise the horizontal scroll bar.

Requiring obscure 'Plug-Ins' as essentials to effectively use your site.
A Plug-In is a piece of software that works in conjunction with a browser to display specific material, e.
g.
a movie clip, sound, etc.
As your visitor probably won't have them, they then need to download the Plug-In and install it before they can continue using your site.
This is a guaranteed annoyance.
Often people do not know how to do this.

Persistent little 'Pop-Up' screens that appear on top of the web page, particularly if they contain advertisements.

On subscription and order forms don't ask for excessive information.
Research indicates that response drops off almost proportionally to the amount of information requested.