They Promise the Moon and Deliver only Moonshine
Most directories, forums and social bookmarking sites require one to login before posting links and often with a password or using a captcha to catch spammers, so it's very unlikely that a link building service can automate this process without some devious means to gather a lot of links quickly.
Usually they buy up a bunch of low quality websites, forums or directories (which are a dime a dozen since Google removed the PR from most pages with outgoing links a few years ago). Then they use those to post your links and being a network they rarely produce much benefit because Google is a domain registrar and knows which sites are involved in networks. So, while you may gain a lot of links Google may consider them pure junk. This could also reflect on Google's view of your website, i.e., harm your Trust Rank or quality score.
They Promise to submit to Multiple Forums and Social Bookmarking Sites
Most forums and social bookmarking sites automatically put No-Follow on the links to prevent spamming so any links you get from those sites should be for the purpose of getting visitors and not link value or Page Rank.
They Guarantee High Quality One-Way Links up to PR 9
A lot of link building services guarantee they will get you one-way links with high PR. However what they don't mention is while the home page may have high PR the page the link is placed on likely doesn't. The only way to get decent PR is to request someone link to you from an already established page with PR, as new pages have to start out with PR 0 and work their way up and it can take years to get decent PR out of new pages.
They Only Submit to Do-Follow Sites
Even if they say they only submit to Do-Follow websites this usually won't help you any because unless a site puts No-Follow on outgoing links Google usually won't allow that page to gain any PR but then the link won't pass any "link juice" etiher.
They Provide Mega Links - You Get a Mega Penalty:
They often guarrantee they follow Google's guidelines and at the same time promise to submit your site to hundreds of sites which makes the former statment an outright lie. Google keeps a record of how fast your website is growing so if you suddenly gain hundreds or thousands of links it will throw up a red flag and likely cause your site to be penalized or filtered out of existence in search.
They Claim their Own Website has Thousands of Links Using their Service
However they often distort the truth. One link building business stated they had "100,000 +" links according to Yahoo Explorer, yet if you check it they actually only had 431 links. However if you checked Yahoo search engine for their business name (3 words) it does indeed show up 100,000 links, however, what most people don't even realize is that those are pages that have one or more of those words listed somewhere on the website, not necessarily all three words together in that exact order and often Google replaces synonyms for the words you specify anyway which further inflates the word count.
They Claim They Can Get you High PR Links
However, if you check the links to their website you will probably find that they don't have any High PR links pointing at their own website. Most SEO explorer tools list the highest quality links first so that's easy to check out. If a site can't gain High PR links for themselves then it's not likely they can get it for you either.
Their Testimonials Lie
One testimonial on a link building site said they had been using the link building service for 4 years yet the domain was only 3 years old. Something wrong here! One site had over 60 testimonials yet only a few of them were linked to websites so you can verify their claims and the ones that were linked weren't ranking above page 10 for their main keywords.
Summary: If a submission service lies as much as the samples above do you want to trust them with your money?