The way I understand it, this series is more or less testing the market out. This information is a little dated, but I heard about a year ago that it'd be 26 episodes that would follow some of the most popular early story arcs from the manga, and if popular enough they would make it a long series that animates every story arc in the manga (yay!).
Yes, the first episode is different, but there's only two points that are really THAT different: the rest is simply extra - and welcome extra in my point. Why was Keiichi chosen to have his wish granted? We were only ever told that he was the kind of person who qualified: now we have SEEN why he was qualified. A nice guy who has a pretty rotten life :p I don't see why they had to set it one year later though (which means Keiichi is one year older, and Hasegawa Sora is already at Nekomi IT) - it means later that either Megumi will be already at NIT, and as such some good bits will be missing, or she'll be introduced later and a year younger than she was in the manga. A pretty pointless change, in my opinion.
Still, as a big fan of the manga, I'm excited by this: the looks and sounds are just what I would have hoped for, and so far there's no dramatic deviations from the manga (though I'd imagine some things that were dropped later in the manga, such as the variable-sized sigil on Belldandy's forhead, will never make the anime). The OP was good to see too: just a brief snippet of characters we'll see more of later. Roll on the first Mara arc :D