ShinHakkaider
Hero
So Astro Boy and Cowboy Bebop both get to be anime, but Thundercats doesn't? Anime is Japanese for "animation." Thundercats was animated in Japan. While it may not have a lot in common stylistically with the stereotypical Japanese approach to animation, I don't see the relevance... "anime" spans a vast range of styles and tropes anyway.
So when someone says, "Thundercats isn't anime, it has the wrong look," my first thought is, "This person is not familiar with a range of Japanese animation."
I'm familiar with the wide range of japanese animation and have been into anime for about 30+ years. There were several american shows produced during the 80's which used TMS and Sunrise (Nippon Sunrise at that time) for their animation. You dont really see it in the animation used IN the shows as much as you see it in the OPENINGS of those particular shows. Most notably SILVERHAWKS, THUNDERCATS, BIONIC SIX and MIGHTY ORBOTS. The average animation during those shows were actually pretty crappy. Now I know that this is neither here or there but when I was a kid and saw these openings I thought that they were amazing. But I'm one of those people that dont consider anything produced for an American audience to be Anime even if it HAS Japanese animators working on it.
Both the GI JOE animated movie and TRANSFORMERS THE MOVIE were obviously japanese animated and were fun films. The story telling sensibilities of both came the closest to American anime as anything is going to get (with the obvious exception of AVATAR THE LAST AIRBENDER) but still is not, to me anyway anime.
Your example of ROBOTECH is a bit of a cheat. As Carl Maecek got the rights to three different shows (SUPERDIMENSIONAL FORTRESS MACROSS, SUPERDIMENSIONAL CALVARY SOUTHERN CROSS and GENESIS CLIMBER MOSPEADA) edited and reworked the dialogue and story to make one big and completely different story. Sandy Frank did the same thing with KAGAKU NINJA TAI GATCHAMAN when he turned it into G-FORCE. Are those ANIME? obviously yes, but they're butchered forms of it. It's the same as if someone took all six of the Star Wars movies, re-edited them and re dubbed them so that Darth Vader was the good guy and completely mis understood and those STOOPID rebels were in the way of him being reunited with his long lost shiny Droid and his short rotund friend (oh and his kid). Is it still Star Wars? Sure. But is it still STAR WARS? I wouldn't call it that.