The 'hard' factors are sometimes dismissed as quantitative, as if they could not be used for quality measurements. This is very untrue. Not only do they contain valid markers of scientific quality, they are essentially all we have got that can be measured in this regard! Take for example publication volume. Within this simple parameter, several important quality factors are hidden. Someone with a large publication volume (many scientific articles) has proven him/herself to be tenacious and productive. It implies for example the ability to aquire funding to be able to continue for so long. It also implies good networking skills, and/or the contribution of important reagents or know-how to the scientific community.
Another important factor is the quality of journals. Sometimes bad articles are published in good journals, and vice versa. However, this measurement is here to stay, and in general has a lot to say about the scientific quality of a given work. Most reviewers gage the quality of an applicant by quickly browsing through their publication list, looking for excellent journals and mediocre journals.
Citations are of course also important. This parameter conveys how often individual articles are cited by others. Here it is important to note that bad articles can also be cited. However, bad articles tend to peak, and then decline rapidly in citations. Good articles are more likely to remain on a constant or increasing level for a long time. So, on average, for a given point in time, high citations for good articles will always outshine bad articles.
Moreover, parameters such as citations and percieved journal quality are at least intuitively used by all external reviewers, and influence the result of a quality assessment.