Matt has written a compelling post on the topic a few years back. Brian and Rachel released a video on the reconsideration process, too. If you are not familiar with these sources yet, definitely have a look before submitting a reconsideration request to Google.
What is left to say? After being on the reconsideration team for some time I’d like to share some personal thoughts.
Malware
Hacking and malware distribution seem to be more common these days. If your site has been labeled in Google search results as potentially dangerous, you’ll need to clean it, fix the vulnerability that caused the problem and submit a request for malware review, not a reconsideration request. These are two different processes because they are different issues. The malware review is fully automated and usually processed within 24 hours, although I’ve seen some requests processed much faster. Make sure you read all the information on malware detection and test your site with Googles safe browser. Remember though that there is no need to submit a reconsideration request.
Technical issues/downtime
If you’ve experienced technical problems like server downtime, this can have over time an impact on your sites visibility in search results. If your site returns server errors like 503 all the time, it might be crawled less often for example. While fixing the issue is worth the effort, there’s no reason to escalate the situation to the Google webspam team. Neither are any review and feedback requests via the reconsideration request. There are dedicated channels for webmaster communication, most importantly Google Webmaster Help Forums. Take advantage of the wisdom of the crowd and use them for feedback. Please do not submit a reconsideration request as there is likely no need for it.

If your site looks anything like this keep on reading before submitting a reconsideration request.
Parked/expired/previously spammy domain
Say you’ve just bought a great, old domain. Yay! Check the site: operator and see if it’s indexed. If it is, you might as well focus on developing your site. If it isn’t I’d consider checking the web archive to see if the site has a dodgy past. If that’s the case, it’s crucial to develop the site and add some value before submitting a reconsideration request. Do not submit a website that is still parked, an empty or content lean/scraped content/low quality content site. Give Google and users a good reason to like your site again and show the effort to develop content and add value and only then submit a reconsideration request.
Spammy site
Say your site falls into the obvious spam category (scraped content, linkfarm, you name it) and you got caught — don’t waste your time submitting a reconsideration request, unless you made your best faith effort to clean everything on and off the site to bring it into compliance with Google Webmaster Guidelines. However, if you have got a quality site that has been in violation with Google Webmaster guidelines, there is a good chance you have been notified about an issue via webmaster tools. If you’re checking your webmaster tools account sporadically, set up message forwarding. Read the message from the webspam team carefully, they usually include valuable information regarding the nature of the problem. Once it’s fixed, submit a reconsideration request. Make it brief and to the point. Don’t apologize, there’s no need for that. Neither is there any reasons to resort to anger or profanity. Describe what has been changed to solve the problem since the initial warning. Also, feel free to write in your native language, you will be surprised how many languages the reconsideration team covers. Or in one you are comfortable with. Please keep in mind that there is no need to resubmit reconsideration request until you have received a confirmation that it has been processed, redundancy does not help or speed up the process. Also please note that it is crucial to make sure to fix the problem properly. I’ve seen webmasters with decent sites trying to see if they can fix their site just a bit and get away with it. Removing some of the hidden content or maybe making it just somehow visible? Not the best idea. It is important not to play games while submitting a reconsideration request.
Commonly asked questions
Last, I have heard several questions during my interactions with webmasters at conferences and site clinics. Below I will address the most common and the most frequently asked ones.
Q: What if I don’t submit a reconsideration request? Will my site be penalized for ever?
A: Penalties typically time out, eventually. But it could take time. It’s probably better to submit a request if you’re running a quality site.
Q: My site seems to have dropped within the last few hours/days. Have I been penalized?
A: Google Search results are in a constant flux. Hours or days are usually not significant time frames from a search engine perspective. Wait and see how the situation develops over time. Don’t submit a reconsideration unless you believe you have violated the Google webmaster guidelines in which case fix the problem first.
Q: I have received a message after submitting a reconsideration request that says the site was still in violation with Google guidelines. What can I do?
A: Don’t resubmit another request right away. Read Google Webmaster Guidelines again. Take a long and critical look at your site, the quality of content, your redirections, your outgoing links, etc. If you still have no idea, consider asking feedback on the Google Webmaster Forum.
Q: My brand new site has been doing well and now it dropped rapidly. Why has a new site been penalized?
A: Unless you’re doing something spammy, it probably hasn’t. It takes a bit to evaluate all signals of a new site and fluctuation in natural search results are common.
Q: I have received a message after submitting a reconsideration request that says manual action previously taken on the site has been revoked. But my sites visibility in search results does not change — why is that?
A: Keep in mind that updating the Google index may take some time, so sit back, relax and check again later.
Q: I hear that my competitor submitted a spam report saying I was spamming. Do I have to submit a reconsideration request?
A: Unless you actually were violating the Google webmaster guidelines there is no need to submit a reconsideration request.
Q: I am 100% sure I’ve not done anything spammy on my site, yet my Toolbar PageRank dropped. Have I been penalized?
A: You can always check the message center in Google webmaster tools for potential warnings. But keep in mind that the Toolbar PageRank indicator is just that: an indicator. The actual and live PageRank your website has is continually updated in the Google index and is not visible. The best advice I can give is to not worry about the indicator and to keep your focus on further developing your website and create great and compelling content for your users.
Having addressed all these questions, I hope this helps you better understand when and how to submit compelling and successful reconsideration requests for quality sites you own. Please leave a comment if I have missed out on anything and I will try to address it.
Update: Make sure to read this interview with Tiffany Oberoi talking about reconsideration requests.