The business of optimizing websites for search engines is an immensely difficult, convoluted task. It requires a massive amount of knowledge, an incredible dedication to keeping up with one of the fastest moving industries on the planet and a ruthless addiction to testing and analytics. Here's my list of some of the reasons SEO/M is so challenging (not that I don't love it, of course):

• Search Engine Guidelines & Offerings
  The search engines constantly refine their parameters for inclusion, acceptable practices and even display properties. In the past 2 years, we've seen major events like nofollow, the introduction of Webmaster Central, new guidelines on paid links, policies related to supplemental results at Google, the launch of Yahoo! Site Explorer, the Sitemaps program and dozens more. Only yesterday, Yahoo! came out with a new parameter to help eliminate the usage of the Yahoo! directory data from a SERPs listing.
• Crawling & Accessibility
  Dynamic parameters, frames, non-HTML content (PDFs are indexable, Word documents are, too, but Flash is still on the fence and a lot of Java and AJAX doesn't function at all), factors determining crawl depth (inlinks, subfolders, fresh content, duplicate meta data) - all of these change regularly and require constant attention.
• Duplicate Content
  Penalties, page exclusion, supplemental results - all these await the site that doesn't carefully monitor its content. Not only do you have to pay attention to copies of content on your site, you must also be wary of others who might re-publish your works, licensing systems, content sharing, scrapers and the value in letting others excerpt or copy your work (and how to create proper attribution and spider instructions).
• Keyword Research
  With more than a dozen different tools providing search keyword data, what do you trust? How accurate are the estimates? How do you run successful test campaigns? What keywords show the right intent, focus, relevance? How deep should you use KW research to build long tail campaigns? The questions keep on coming, even on a basic issue like keywords.
• Changing Algorithms
  From the sandbox to the trustbox to the influence of keyword usage and links, we see shifts and advancements in algorithmic rankings constantly. Five years ago, it was all about keeping one step ahead of those changes. For many folks now, it's about predicting how advanced the engines might be 2-4 years from now and shooting for the perfect site to fit those sweet spots.
• Controlling Spiders & Sourcing
  301 and 302 re-direct rules and interpretation, meta robots tags, robots.txt and directives that perform "legal" forms of cloaking (showing different content to engines and users) all demand careful attention and proper discretion.
• Emerging Traffic Sources
  Digg, Reddit and StumbleUpon now drive huge quantities of traffic, as do blogs, video sites and new forms of ads (like paid reviews). Ignore these sources and your competition will get a leg up on you in the future.
• Conversion Rate Optimization
  Getting visitors to the site is only half the battle. Search folks are often required to guide clients through the process of optimizing a site for the users as well. From copy to information flow to user experience, design and even product quality, the responsibility can be massive.
• Web Design
  Though some see this as subjective, the issue of professional design is critical. Search marketers must put a critical eye to the layout of a site, the organization of elements on the page and the visual appeal or risk losing visitors, links and credibility.
• International & Multiple Language Issues
  The search engines currently do an awful job with many International issues, particularly when targeting multiple languages on a site or when offering multiple sites in different countries with content in the same language (see duplicate content above). Navigating this minefield is daunting, to say the least, and worst of all is that moving from country to country can often mean targeting different engines or traffic sources based on what's popular in that region.
• Localization & Geo-Targeting
  Even if you're just optimizing in one country, local search results, local engines, yellow page style directories and geotargeting for ads or content can make for trying work. The experts in these arenas apply their own specialized knowledge of how to get the most for businesses seeking regional clientele.
• Usability
  Much like web design, usability can impact all of the key elements that drive traffic to and through a site. At SEOmoz, a major part of all our site reviews is usability; it's the more human-focused sibling of search engine accessibility.
• Appealing to Linkers
  Every website that requires SEO must appeal to two groups of users, those who will buy/read/interact, and those who have the power or ability to link (and spread the word). Your used car dealership in Detroit may not need an AJAX embedded map, but it sure does help when trying to attract natural links from the blogosphere.
• Content Creation
  Creativity is required, as are writing skills and experience in building compelling web content. You have to know your users, your niche and target your content appropriately. I starts with great brainstorming and ends with great execution of language, images, video, audio and interactive content.
• Web Analytics
  What actions do you track? How do you measure success? Is A/B or multivariate testing required to get the most out of your campaign? What can the hundreds of trends and data points tell you about how your site can be improved? How about latent conversion tracking, offline campaign coordination or dayparting?
• PPC Ads
  Think buying ads on Google, Yahoo! or MSN is easy? Think again.
• Banner Advertising
  Are banner ads right for your site? Are they a good way to monetize short-term traffic? Can they help to bring you qualified leads or brand the right demographics to buy from you later?
• Contextual Ads
  Will AdSense earn you revenue? Is YPN a better deal? Will choosing contextual ads lower your relevance or hurt your chances to gain inbound links?
• Search Results Pages
  The darn things change all the time - from maps for local results to separate boxes for flights or recipes or weather to query refinements, suggestions and more. If you're aware of all the possibilities early, you can often take advantage of the modifications for rankings benefits, but if you miss the boat, it can be a struggle to get traffic even if you're a top result.
• Monetization Strategies
  Advertising? Freemium? Paid content? E-commerce? Consulting? Paid reviews? Not only do search marketers need experience with how to monetize a site, they also need to be able to identify the best choices and weigh the value of certain tactics with long and short-term goals.
• Blogging
  Organic SEO today often requires considerable expertise in the blogosphere. Sometimes, you're not optimizing a blog for your own site, but rather, figuring out how to leverage the power of blog traffic, links and reputation to build your brand in a niche.
• Offline Marketing
  You may not need to be an expert, but you at least need a solid foundation on ad agency tactics, public relations and guerrilla marketing to properly integrate these campaigns with your online approach.
• Copywriting
  There's a whole blog on the subject and plenty of folks who struggle to implement it properly. You may not need a Journalism degree, but it certainly doesn't hurt to have the background (like our glorious leader).
• Keyword Usage
  Once upon a time, the right number of term repetitions in your content would bring you search success. Today, more advanced search engines have made density formulas and keyword stuffing obsolete, but usage remains an issue for sites large and small. Using keywords in the proper places - titles, headlines, URLs and anchor text still has a big impact, but care must be taken to avoid cannibalization and prevent the search engines from interpreting your optimization as spam.
• Web Hosting
  Even something as basic as hosting requires some SEO knowledge - Russell Jones explains here.
• Link Building
  From link requests to natural link acquisition strategies to partnership networks and paid links, the adept search marketer requires an enormous bag of tricks to make link building campaigns consistently successful.
• Viral Marketing
  It's a hot topic, and requires serious effort. Linkbaiting isn't all fun and games - it takes trial and error, proper targeting, recognition of the various social audiences and a strong dose of content creativity and ability to execute.
• Social Media
  From MySpace to Flickr to LinkedIn, Squidoo and Yahoo! Answers, social media can't be ignored. Traffic, leads, networking opportunities and links are all part of the magical world of SMM.

No one is an expert in all of these; as we've proven in the past, SEOmoz itself has severe weaknesses in PPC, banner ads and contextual ads, and we're still struggling with monetization strategies and occasional changes in the SERPs (particularly those pesky OneBox results). Don't let anyone tell you your job as a search marketer is easy. The sheer amount of expertise required to provide great results to clients, companies or projects of your own is staggering.

I'd love to know - what part of the SEO/M job challenges you the most?

p.s. I believe Danny did a similar post when he defended SEO a few weeks back.