After that, all the uniqueness ended. The Mugan were all the same looking and they were boring, ugly CGI cannon fodder enemies, the Anti-Spiral mechas were similarly boring and a lot of the shounen original attacks disappeared. Arc GL for instance only had one new move, and it was just a reall strong punch. CG GL had a cool design and Giga Drill Breaker, but it didn't do anything new either. And TTGL's attacks aren't very creative - how many "multi combo" entities in previous shounen shows have all the weapons of certain people combined? Like Apocalymon from Digimon for example?
All the mecha upgrades also happened within a few episodes of themselves. It took ten episodes to get to Arc GL and 2 to get to CGGL. Then 1 to get TTGL. I couldn't find myself excited when TTGL stepped out of a galaxy because it was so sudden and didn't make all that much sense, given how much had to be explained about Arc and CG.
The final fight was great because Anti-Spiral went insane, but otherwise I couldn't wait for it to end and see how the resolution went. Partly because I was expecting a Gainax end, another part because I knew the heroes would win. The fight against Lord Genome was a lot more intense because Dai Gurren was destroyed and Gurren Lagann was BEATEN DOWN, and Simon beating Lord Genome came as a surprise.
Overall, the only interesting parts of the final arc weren't the battles, they were the characters and their various relationships. That's really what held me through - the action felt hollow/meaningless even if it wasn't meant to be.