^I like the Flood too and I agree that the whole second half of Halo 1 involves fighting them. To me, there's just a qualitative difference between fighting the Flood, the Flood, and more Flood for hours on end versus fighting the Covenant, Flood, and Sentinels all at the same time. The Library was 100% the former (and not fun in my opinion), the following missions were the latter (and were considerably more fun).
Either way, hopefully Halo 4 will be able to recreate some Flood vs Covenant vs Sentinels vs Master Chief battles in some way - those got pretty crazy in Halo 1 and I look forward to hopefully fighting some more.
A side note on the battle rifle scope thing, anybody else find it odd that both the battle rifle and the DMR will be appearing in this game? I always assumed they were more or less the equivalent of one another between 2/3 and Reach and including them both seems somewhat repetitively redundant over and over again.