I agree that the Borg aren't as scary as they used to be, and if I didn't have such terrible memories of being frightened by those damn scary Borg as a child, I'd want the old Borg back too. Understand, though, that what the Star Trek writers have done to the Borg, they've done to every villanous race that they've ever created. Here's the formula:

1. First, create an enemy species/empire. Make it powerful, incomprehensible, and foreign. Not only that, but evil badasses. For example
  • The Borg- they're incomprehensible because the whole concept of a hive-mind is one that's almost impossible for us to conceive of. Also, as a collective, they represent the Soviets, so that the entire Cold War indoctrination regimen can be recalled against them.
  • The Dominion- Appeal to our paranoia; They could be anybody...
2. Spend a couple of episodes, over one or more seasons, having said enemy establish its power and fearsomeness. You can also do this by providing background information. For example
  • The Borg- undefeatable, scary with those goddamn nanoprobes hidiously disconfiguring their victims. Wholescale slaughter- Wolf 359.
  • The Cardassians- Smooth and sinister Nazi style concentration camp backstory provided at the beginning of Deep Space Nine.
3. And then- deconstruct! Take an individual from the offending alien species and present him or her as vulnerable and cuddly, or at least heroic and noble. For example In fact, this patten is often manefested in micro- form in one or a short series of episodes. Star Trek has always been preachy, and its overstated message tells us not to hate, because the enemy is just like us. However, naturally, its also fun to be scared shitless, and fun to triumph and blow things up and not feel bad about it. Hence, screenwriters constantly churning out new enemies for us to hate and fear.

Chiisuta makes the perfectly reasonable comment that the above progression may be unavoidable: "the less we know about an enemy, the scarier his is. but it would be impossible for the next gen kids to keep running into the borg and not learn anything about them. the more you learn, the less frightening they are. of course we discovered weaknesses. this is especially true of science fiction."