I just noticed your implication. So, which am I; inactive, a lurker or a troll

Perhaps you noticed an implication but not one of mine? At least that must be the case if you take it as an implication personal to you. I responded to MugaSofer's question from the recent comments thread and so didn't know any context that pertains to yourself.

As a literal answer to your question I assume that your weeding vs commenting ratio must be higher than average. That would place you somewhat towards the 'lurker' end of the spectrum. That needn't be considered offensive---it indicates having other higher priorities and the ability to restrain oneself from impulses to waste time arguing on the internet.

, and why does it mean my votes shouldn't count?

I approve of the downvote limitation by karma as it stands (unless a new, smarter system is coded). It isn't a perfect indicator of how much consideration should be granted to a HTTP request representing a click by certain account but it is adequate for the task. If I were personally allocating the right to vote on an individual basis then I would grant you an unlimited supply of votes since from what I recall of your comments you meet the requisite standard of having-a-clue. But I don't consider the automated and rather trivial system of raw karma totals to have the necessary information to make that judgement so are not especially outraged that you reached your quota.

The reason why accounts with low karma are limited in how much they can vote is because the anthropomorphised karma system doesn't feel like it knows enough about the quality of your thinking to be comfortable granting you the amount of influence you are attempting to have by enacting judgement. Fortunately it isn't exactly a complicated or strenuous task to establish trust with the karma system by writing a few comments or pasting in some inspirational quotes. In fact, anyone who did find it hard to work out how to gain karma by making comments is not likely to be the kind of person whose votes I would find personally useful.