My initial thoughts on this new entry in the Arkham line: less Ameritrash, more Euro.
FFG has long been known for its entries in the Ameritrash category, and Arkham Horror was first in line, with all the requisite features: big board, long play time, mechanics loaded up with "chrome", and lots and lots of fiddly bits. (Granted, Arkham Horror was initially a bit light on the mountains of plastic bits that Ameritrash games are known for, but that oversight has subsequently been corrected with the release of the miniatures.) And expansions - lots and lots of expansions. A veritable Tides of Iron for the lover of Lovecraft.
Eldritch Horror seems to be moving away from all of that. The board is smaller, the mechanics seem tighter, the fiddly bits fewer, and plastic is nowhere in sight (although I am confident FFG will be willing to sell some suitable plastic bits to the diehard if-it-doesn't-look-like-Axis-and-Allies-I-won't-play-it crowd). The preview focused on what appear to be carefully designed, interlocking mechanics - more the mark of a Euro design than an Ameritrash design.
One significant variation from the Euro trend would be more emphasis on the flavor - which I think is one thing FFG does get right. Elder Sign (another move away from Ameritrash and toward Euro in design) was criticized for being light on the flavor, and I think they've taken that to heart.
I don't really have a specific thought on whether this is designed to replace Arkham Horror - I think it's more designed to draw a wider range of gamers in. Gamers who would look at a gigantic layout with massive piles of cards and bits and would instinctively say "no thanks" may take a closer look at this one, and I think that's the goal. FFG has no fear of endless expansions - just look at the LCGs - so its presence doesn't automatically mean no more for Arkham Horror.