In many respects, your LinkedIn profile should be treated in the same way as your CV, so don't include any personal details that could potentially count against you. That means that your age, marital status, religion etc are not usually needed.
The distant past
Employers will be mainly interested in your recent past, so focus on it. Of course, you can mention old jobs that were particularly impressive, but try to keep the majority of your profile centred around the positions that you've occupied within the last ten to 15 years.
A poor photo
Using a small / out of focus / dated image is a pretty bad first impression to make. You (hopefully) wouldn't turn up to a job interview without attempting to look presentable, so why should your LinkedIn page be any different? You don't have to be photographed in your finest suit, but make sure that you look at least vaguely respectable.
Revealing your salary will never end well. The chances of an employer wanting to offer the exact figure that you're looking for is slim, so giving away your salary details will either lead to you underselling yourself or pricing you out of the job. Don't give anything away - everyone loves a little mystery.
Clichés / overused words
When recruiters search through the 200 million profiles on LinkedIn, a lot of candidates will inevitably merge into one. Certain words such as ‘creative', ‘innovative' and ‘motivated' might sounds impressive, but they're so overused that they're unlikely to impress anyone.
The trick is to be a bit more creative - rather than just listing skills, try to display how they've benefited your previous employers.
Previous job disputes
Don't provide details of why you left a previous job - all that's going to do is create a potentially negative section of your profile. Your LinkedIn page should be about selling yourself to employees, not pointing out your negative traits that previous bosses didn't like.
If the employer wants to delve a little deeper, then let them do that at the interview stage, where you've got the chance to impress them.
It may be obvious, but it's worth being reminded - check your profile through a few times for any spelling or grammatical mistakes. Your LinkedIn profile is basically a mini CV, so show that you've displayed the thought for detail that your future boss would want you to show in your new job.