As an occaisional reviewer, I thought that it would be good to outline my personal preferences as a consumer -- so you can guess at my biases. My gaming purchases usually come in spurts of the times when I get to visit a good gaming store, such as Games Plus in Chicago, The Compleat Strategist in New York, or Gator Games in San Mateo. I would split up purchases into core systems, sourcebooks, and adventures.
There are very few systems which I am likely to play "as-is" without significant homebrewing or changes. If the core system is tied to a setting, chances are that I will either not like the system or not like the setting. If it is a universal system, chances are that I will want to tailor it to my tastes and to the particular genre that I want to run. Consequently, I tend to buy core systems to mine for ideas. For these, a few good subsystems or an interesting background matter more than overall quality.
My ideal system is easy to play and easy to understand, especially for newbies. The challenge of a core RPG is to be intuitive not only for resolving actions but also for how adventures are designed. A good system doesn't specify rules in a lawyerly fashion, but instead relies on common sense. The problem for newbies is not "how does dodge work" (which can easily be decided with a bit of common sense) but rather "how to I run or play through an adventure". Long-winded nitty-gritty explanations are unlikely to be read in the first place and are quickly forgotten during play. RPGs must and should require assumptions, as opposed to trying to provide a complete reference to be consulted during play. Attacks of opportunity D&D (3rd ed) are a good example of this. The rules try to spell this out by listing every possible action and whether it provokes an AoO. Everyone was confused by this, due in part to an erratta-corrected phrasing. I think the real mistake was in designing the rule to rely on minutia of phrasing rather than relying on common sense.
Backgrounds are very much a matter of taste. I prefer backgrounds which involve real-world cultures and information, as opposed to pure fantasy -- such as historical or modern-day settings, alternate histories, or near-future science fiction. This provides a lot of rich detail for very little cost. While I rarely play sample adventures, I still find them very useful as a demonstration of what the designers intend play to be like. The adventure usually tells me much more about a game than many pages of designer's notes or GM advice essays.
My favorite systems tend to be older. More recent RPGs seem to tend towards long explanations and less ready-to-play material. My favorites include Ars Magica, James Bond 007, Paranoia, and Call of Cthulhu. One issue is that usability seems to have suffered in later design. Around 1990, it was pretty standard for an RPG or RPG supplement to be in a boxed set. The basic set would have several books along with dice, a fold-out map, character sheets, some handy reference sheets, and a real introductory adventure. As a bonus, GM material like the introductory adventure would be separate from the players book -- so you can lend the rulebook to new players, say.
Sourcebooks I look at for actual gaming use, so signal-to-noise ratio becomes important. A good index and bibliography (with complete information) are invaluable. A glossary can be nice but isn't so important. I find that good, detailed research is very important for me. As an example, I picked up a number of mystic conspiracy sourcebooks for my Enclave game, including "Secret Societies" (for Nephilim), "GURPS Voodoo", and "The Quick and the Dead" (for Wraith: The Oblivion). The former two were very useful to me, were well-researched, and had good indexes and bibliographies. I used some "GURPS Voodoo" stuff straight, while "Secret Societies" was mostly mined for ideas and its essays on organization and beliefs of secret societies in general. "The Quick and the Dead" I found very little use for.
In general, attention to detail and good characters is priceless. To me, ideas are a dime-a-dozen: research, detail, and good supporting characters are ideal. Some sourcebooks have bits of fiction in them, but I usually find it unrewarding unless it is something I can photocopy and hand out to the players as stuff their characters would read.
As a rule, I tend to pick up any of TSR's historical campaign series that I find as they are generally excellent. GURPS sourcebooks are of variable quality overall, but some are real gems (like "Fantasy II", "Alternate Earth", "Voodoo", and "Goblins"). HERO sourcebooks are not so good for me -- I sometimes buy them because I often use the HERO system, but I find I actually get more mileage out of books for other systems. An exception is "Normals Unbound", which was terrific -- lots of ordinary characters so that a gang member or policeman that the PC's meet have depth.
Adventures I buy looking for ones that I can adapt into my campaign. I typically find that very few games have adventure formats that I like. The best adventure books in my experience were for Call of Cthulhu, Daredevil, and James Bond. The format and quality to me are a cut above most others I find.
Again, what I look for here is usable detail. Good maps are invaluable as are good ready-to-use player handouts. In recent years, usability of modules seem to have suffered. It used to be common to include things like fold-out maps, a separate booklet of stats, hand-out-able illustrations, and so forth. However, this seems to be less common. Pictures usable in-character are invaluable, along with full journal entries -- printed so that I can photocopy them and hand them out directly. Flavor illustrations such as a picture of hypothetical PC's fighting with opponents are essentially worthless to me.
Adventures with extensive plot outlines (like those for the Torg or Masterbook lines) are less useful to me -- not so much because I am against plotting, but that the plots are not useful after modifying to take into account my campaign and PC's. Characters, maps, and handouts are more easily adapted. Short, poorly-detailed adventure "seeds" are a dime-a-dozen and I usually find them essentially worthless.