1. You may think diversity in research counts, but it does not, at least not for tenure: This is the age of interdisciplinary research where an ability to transcend boundaries is key to solving important scientific problems. Thus you may think that a track record of having worked on diverse projects may help. Apparently not for tenure; tenure committees still seem to be more impressed with hedgehogs rather than foxes. They don't want a "dabbler"; they want someone who has proven his or her expertise in a single, narrowly defined area of research. Personally I find this approach disappointing, not only because thinkers working on diverse projects can enrich a department but because scientific progress itself needs all kinds of tinkerers, from the ones obsessed with a single problem for fifteen years to ones having their fingers in several scientific pies. Sure, being able to probe to the core of your chosen speciality is important and in fact is indicative of sustained scholarship, but the capacity to think outside the box and apply your knowledge to a variety of problems is increasingly important. The way I see it, tenure committees seem to be stuck in the transition period between the age of specialization and that of diversity. In twenty years perhaps they will start to appreciate diversity more, but for now, the lesson seems to be that narrow specialization is much more important, at least for getting tenure. Once you get tenure of course you can break free of such constraints.
2. Interests outside actual research don't count, and paradoxically, interests related to research may harm your prospects: This point was even more revealing. Yes, you can have hobbies (thank you!), but the more unrelated a hobby is to your research, the more benign it will seem to the tenure committee. The good news is that cooking and horse-riding are good. The bad news is that blogging and textbook writing are bad. If you are blogging in an area related to your work, there is a much greater chance for the committee to think that you are wasting your time which could be more fruitfully spent in actual research. Similarly, textbook writing will be frowned upon, even if you write a best-selling textbook. Basically any time away from research by definition is time that can be spent on research, and that's how tenure committees think. This is not too different from the attitude that certain PhD advisors have toward their unfortunate graduate students, but that's how it is.
However, there are probably ways in which you can try to put a positive spin on your blogging and other activities in a way that makes the committee appreciate your efforts in these areas. Recently I attended ScienceOnline2011 and there was a session in which tenured professors who thought that their blogging actually helped their tenure process gave some valuable advice on pitching blogs in tenure applications. First of all, try to convince the committee that blogging is not just a pastime but a valuable way to communicate science. Thus, you could possibly make a good case that skills gained from such communication could and do help you in the classroom. Secondly, try to convey the impact of your blogging on department visibility by citing references to your blogs in the media and in scientific journals. International citations could especially help. Ultimately, however, I don't think any of these strategies would work in the majority of cases since again, none of these activities contribute to actual research as much as they do to teaching and outreach, aspects of science that are usually considered less important by tenure committees.
Are you depressed yet? Well, all this is probably not as unfair as it sounds. Think a little from the perspective of the tenure committee. They are going to run the risk of hiring someone who will hang around for thirty or more years and become a permanent department fixture. Thus they want to be absolutely sure that they hire someone who has demonstrated scholarship (and funding potential). The fact is that if you can prove your mastery in one speciality, the committee can be more confident of your potential in tackling other complex problems. They want someone who can do sustained work in a single area for half a dozen years and bring in the bacon.
At the same time, tenure committees need to awaken to the new reality where an ability to appreciate and work in diverse disciplines is as important as the ability to delve deep in one specialty. As for blogs and textbook writing, while I find the attitude disappointing, it again makes sense. Departments hire you first and foremost for your research and publishing potential. They may treat your blogging and other related activities with mild interest at best but one cannot blame them, at least in the first few years, for relegating such activities to the side when it comes to considering you for tenure.
It is also worth noting that the caveats listed above mostly apply to research-oriented universities. Blogging, textbook writing and diversity may all be appreciated more in institutions equally or more focused on teaching. But there it is; a picture that's disappointing but sensible in its own way. Say goodbye to your utopian childhood impression of science as a career in which you are free to pursue any line of activity and work in any area that interests you. At least until you get tenure, you will have to stick to a narrowly defined set of constraints and toe the line.
After that the world's your oyster. Almost.