My name is Aaron and I've been a professional artist for about 5 years now. I've reviewed many portfolios in my spare time and for hiring purposes. I've been apart of the artistic online community for a number of years and there are some suggestions I'd like to offer, based on the most common things I personally find irritating and could cost you a potential job.
Employers and clients simply do not have time to look at your portfolio. They just don't. There are never enough hours in the day to do what needs to be done. Even if the role they are trying to fill is absolutely necessary it is still an inconvenience to look through hundreds of portfolios. Many employers won't open attachments as the potential for viruses are just too great, so for the most part they like to see online, digital portfolios. Now considering they don't have any time to spare looking at your portfolio, you want to make their browsing experience as fluid, responsive and as fast loading as possible. You don't want the interface to be confusing, you don't want to have to click more than 2 times to start viewing your work and you don't want an image to take longer than two to three seconds to load. I promise that if your portfolio is a struggle to use, and the employer is rushed for time (which is almost always the case) they won't even look at your work and that is a shame because they may have liked what they saw, but they never got that far. Usability is the foot in the door, your work is the sales pitch.
I have seen so many different types of online portfolio but one thing always sticks out is - how quickly can I just look at the persons work? Some are shoddy image dumps like flickr and other image storage websites - these websites have awful loading times, not to mention they aren't the most aesthetically pleasing and while your work is of course the focus, they lack a professional feel. You want to give the employer confidence that you know what you are doing. Some are on the other side of the spectrum, going to town on the design of the website, overdoing the flash elements and creating mind bogglingly complex interfaces with intricate animations. Again, your work should be the focus and unless you are applying for a webdesign role all these elements can not only take a long time to load but they can also be a major distraction.
There are a range of websites you can use, that effectively showcase your work that have excellent interfaces and do not possess exceptional loading times. CGHub is a good example (although it isn't letting me post the link) - this is a concept art community who's users work is usually a little higher in quality than say, places like Deviantart, a great deal of them are professional, industry seasoned artists. An employer, reviewing work on this site will more than likely be familiar with CGHub and therefore will be comfortable with the interface. CGHub also has decent loading times. CGSociety is another option and has more professional artists than most - it also has a stringent quality level that must be adhered to and your work will be reviewed before it is made 'live'. (This is a good indication of whether or not you are at a professional standard) be warned however, that in my experience http://www.cgsociety.org/ has terrible loading times and as I said this may jeopardise your chances of an employer even looking at your work. Deviantart's portfolio service http://portfolio.deviantart.com/ is probably the most effective that I have seen and it is the one I use http://hazzard65.daportfolio.com/ it allows you to set your own URL, it's interface is very simple to use, you can quickly edit the aesthetics, it's very easy to manage your galleries and the loading times are very fast. Another option, similar in execution to DAportfolio is http://carbonmade.com/. Although I have not had experience using it for my own work I know many professional artists that do. It's interface is simple to use and loading times, in my experience, appear decent.
One thing I would suggest you avoid doing is using standard DeviantArt to showcase your work. I love DeviantArt and I have used it for many years, I also know many professional artists that do use it when applying for positions, however the reason I suggest not using it to showcase your portfolio work is because it is strongly considered to be a social media site among other things and it's reputation in the professional world is sketchy. I know several outsourcing companies that won't even open a deviantart link and while that may seem frustrating, the employer is the gatekeeper, you want to please them.
In closing, you want to maximise your chances of success. You don't want an employer to be distracted by any of the pitfalls I described above. So remember these simple points:
Interface - How easy it is to navigate your online portfolio. Are there clear sections. How many clicks does it take to reach your work. Are your contact details clear or easy to find.
Aesthetics - Does the venue for your work demonstrate your professionalism. Does it look like an art gallery, or a warehouse? Is your work the focus?
Load Time - Employers don't have time to waste, how quick are your images loading? Any more than two to three seconds and the employer may lose interest. Avoid flash if you can.