Star Trek: Series V is the working title of the latest incarnation of Star Trek. It is supposed to take place in the 22nd century, which would place it at least 60 years before the events of the Original Series. There's nothing wrong with the basic concept itself, but there are certain problems with it...
Update: As General Wesc points out in his WU, the name of the new series will be "Enterprise", but at this point it is uncertain whether it will actually be called "Star Trek: Enterprise". At least this would give us the excuse to say "Enterpise is not Star Trek".

1.) Star Trek is over 30 years old. During that time many fanfic novels have detailed the pre-TOS history, and have done a pretty good job at it. Naturally, none of these books are canon, but to some people that doesn't make any difference. Sure some of the novels are pretty horrible, but there are also a few gems around. The point is, that there is already a detailed history of the 22nd century. Now, it is pretty unlikely that Series V would pay any respect to this history. They would simply write one of their own, and thus cause major pissed-offness among the die-hard fans.

2.) It is quite obvious that the Special Effects in the Original Series were much more simple than on The Next Generation and later series. If they were to use the latest FX technology to create Series V (which would be the logical thing to do), then it would look much more modern than TOS. That of course wouldn't make any sense.
Update: I recently heard a rumor that the bridge controls and the visual look in general would be more reminescent of TOS than TNG.

3.) Star Trek just wouldn't be the same without Klingons. The first contact with Klingons was made in 2218, so they wouldn't be featured in Series V, unless the producers wish to rewrite that part of Trek history. Even if they were featured, they would probably have ridges in their foreheads, which would lead to all sorts of interesting conflicts. The same goes for Transporters, which were invented in 2205.
Update: The fact that the pilot will feature the first contact with Klingons proves my point. They don't care about what has already been established as fact.

4.) Series V is supposed to take place aboard the Enterprise. This can't be the same ship which was featured in TOS, since she was built in 2218. It would have to be the ship which was displayed in the Recreation Lounge in Star Trek: The Motion Picture, but that ship looked more like a space station. Anyways, why isn't she displayed among the other Enterprises in the Display Case of the Enterprise-D in TNG ?
Update: Apparently, the ship is to be an almost exact replica of the Akira-class (From First Contact and later in the Dominion wars). This is totally ridiculous! The ship looks just like something from the 24th century, not from the 22nd. It's not just the fact that this totally screws up any continuity that pisses me off, but the fact that they didn't even bother to come up with something original is just insulting!.

5.) And finally, what would the writers do if they couldn't trap the heroes in the Holodeck every other episode ?

Update 27 october, 2001: After 5 episodes of Enterprise, I can safely say that it isn't so bad as thought when writing this node. Sure, it has its flaws, but I find myself blissfully uncaring. Let this node stand as a testament that one shouldn't judge something on the basis of rumours alone.