I think the main problem problem with today's F1 is that the races are so much predictable. First of all because of the excellent reliability of the cars(no DNF at the latest race at Valencia). Then because the cars are so even in all conditions, with nowadays' very strict technical regulations(in the past some cars used to be better than others on special conditions, like higher/lower fuel loads, etc, but you don't see that anymore). At last but no least, the cars used to be a lot harder to drive, and the tracks were not as ridiculously forgiving as today, which usually meant that some drivers would crash out of the race.
Result, nowadays the top positions are probably as close as they've ever been... But you still end up with dull races because everyone has a very consistant pace from start to the finish, and there's almost no more human/technical issues.
In the past races would still be thrilling with a very spread out field, because you'd be wondering which ones amongst the top guys would see the finish, and which one would have the extra pace to catch the leaders back at the end.
Tristan has an interresting point of course. But does "blocking" has such a huge influence in races? Even if it was disallowed, people would still have to place themselves close enough to the car in front to pull a safe move, and that's always damn difficult to do because of dirty air. Just look at IndyCar, which has a no "blocking" rule nowadays: it barely has any influence to the racing on road courses, all it does is bring a bit of controversy whenever someone moves to the inside line a bit more than allowed(ie, Viso getting a DT at Sao Paolo, while he actually was trying to catch some standing water, as the track was drying).