Actually there is a huge amount the same. This I found an insult to the intelligence. Ok Onegai teacher was about 10 years before Ano Natsu de Matteru but even so.......
Instead of listing all the similarities and giving spoilers to both anime's I'm going to give you a challenge. Watch both Ano Natsu de Matteru and Onegai Teacher and make a list of the similarities of both anime's in plot, characters etc. I would think you could get at least 20, give yourself extra kudos if you get more. To start you off:
1. The main female protagonists are both red haired aliens.
2. Spaceship hidden in a lake.
3. Both anime's are based in a rural town.
4. Both main male protagonists are wimpy guys with glasses.
5. There is a very perceptive female friend of the male protagonist who has a secret and looks like she hasn't hit puberty yet.
You get the idea!
I should say the the characters are stronger and the romance and artwork is better in Ano Natsu de Matteru than Onegai Teacher but like Onegai Teacher I liked Ano Natsu de Matteru but it just didn't have that X factor that would make me love it.
The poor mark is more for the screenwriter for rehashing an old formula than the anime itself.
