I am not against HFT, but I understand the emotional argument against it and think it has some value.

There are two problems with HFT being carried on by computers. One is obvious: HFT trades done by computer happen super-fast and arbitrage away any slack in the system between bid/ask prices and so on. There's no way a human trader can compete, which means that after a human has made the mental investment to understand how such trades work, s/he is unable to exploit this knowledge and get a foot onto the financial industry ladder. This is aggravating to people who started out trading small-cap stocksor suchlike. Now, day traders are notoriously bad investors to start with and it might be that the market is better off without them, but there's a big qualitative difference between being a bad day-trader who's competing against other humans in the same market, and competing with a machine whose owners have paid extra for it to be located close to the exchange to shave a few milliseconds off execution time. It's like if you put a super-fast mechanical runner in one lane of the Olympic 100 meter dash; while objectively it should have no bearing on the performance of the human participants, which is what we aim to measure, our psychology is not so logical, so seeing a robot complete the 100 meters in, say, 8.5 seconds (handily beating any human for the foreseeable future) would have a massive demoralizing effect on both contestants and spectators. It's one thing to know in the abstract that a machine is much faster than you; it's another to see your best effort radically devalued.

A second reason is a bit more abstract and rarely articulated, but IMHO equally important: human trades are ultimately motivated by human desire. Speculators want more money, but long-term investors want yield to provide a sustainable income, commodity buyers want commodities as industrial inputs or for resale to the public, and people buy commodities at retail because they need to eat, build shelter, and so on. The market ultimately exists to serve human need, notwithstanding the fact that it often serves human greed (which is just hypertrophied need and often correlated with some psychic deficit). HFT and algorithms in general work blindly; as very basic software machines, they operate purely deterministically, with no sense of the context within which their operations take place. Of course, human traders can also lose sight of context (I've heard cocaine is great for this :-p) but even the worst excesses of leverage or psychological dynamics are constrained by the social structures within which they take place; human inefficiency (in decision-making, and in implementation of that decision by others along the chain of execution) serves as a buffer to limit excess, although this is very much a function of scale - see the historical excesses in totalitarian states, for example.

Getting back to HFT and algorithmic trading, what people worry about at the fundamental level is that as it improves over time and more and more money is under the management of computers, none of which have any moral or social context for their trading decisions, our social buffering against excess becomes less and less effective. This is also a reflection of anxiety about late-stage capitalism; logically, we sense that in a purely deterministic market we would eventually see more and more capital concentration similar to the inevitability of a single winner in a Monopoly game, and that's a stressful prospect because we know that such circumstances are not socially sustainable.