Arius was so intense, tough, strong, hard, rough and his attacks were so unpredictable that you could even get afraid of his moves. He even had a magnum and could even trap Dante inside of a black hole...
I've seen some ba...odd opinions on this board, but Arius as a good, no, amazing villain? I fought Arius about a week ago and I was hit only twice on Hard. For crying out loud, the man has a moment where he sits cross-legged drinking wine for a good 5 seconds.
That "flying old man" was more unpredictable and just because you can predict something doesn't mean you will dodge it.
Anyway, DMC1 is acclaimed because 1. arguably the best OST in the entire series, 2. the way EVERY move has functionality (most DMCs have this, but not to the degree that DMC1 has; even Vortex has a use), 3. enemy design (gameplay and aesthetic-wise) and how even mooks can wreck you, 4. atmosphere, and 5. probably being the most difficult.
Only thing I personally can pan DMC1 for is day camera, the inability to charge charge shot and attack at the same time, dat camera, the secret mission windows and locations, dat camera, and not having a detailed enough results screen (I can just look up the info online, but meh).