I'll give some input as a member who is a bit of a geographical outlier here. Improved functionality for searching/filtering past trip reports would be a nice improvement of the site and would be really helpful for trip planning purposes. However, I think if you go beyond a tagging/searching system and actually segregate trip reports into separate forums/sub-forums by region, it will definitely homogenize the sites membership over time. Meaning the regional forums that are strongly represented areas will get the most traffic, discussion, etc, which will attract even more members from those regions to use the site. Members from under-represented regions will start to die off because the line of thinking will be - "ehh nobody has posted in my region's forum in a month, why bother writing up a trip report that will go unnoticed in a forgotten corner of the site?"
The way the forums are structured will have a drastic effect on how geographically diverse the membership will ultimately be. Do you want BCP to be be more of a southwest forum or a broader, more diversely represented forum in the long run? There's no right or wrong answer and that's for Nick to decide.
Just my 2 cents.