Having Green Hill and Chemical, while repetitive, it sort of contributed in the world building the Sonic series has) Level themes: Forces (All the stages look like they belong in a Sonic game while Unleashed looks like visiting an alien planet compared to Sonic. And in my personal opinion, besides the music, I think Forces did a better job than Unleashed in every aspect, for example. Obviously, Unleashed is the one that has more passion, but it makes it even worst for me because of how many baffling decisions it has. To be honest, as flawed Forces is, I really can't find myself to like Unleashed more only because of how that game it is. Imagine Jungle Joyride Day on the current gen, without the frame rate drops. While linear in Unleashed, the day stages have multiple paths at times and overall the scope feels larger, Forces wishes it had Cool Edge or Dragon Road. All these things boil down to charm, something I believe forces lacks. Not to mention the music in my opinion is far superior, from the overworld theme to Endless Possibilities, coupled with the amazing sense of discovery, journey and the amazing landscape it has to offer. The graphics are pretty much on par and this is for a previous gen console in 2008! I genuinely prefer the Warehog to both the avatar and especially Classic Sonic - while it may not be people's cup of tea I feel like it felt so polished with exception of the lack of a drop shadow. I genuinely believe that it does pretty much everything better than forces for a game released 9 years prior. Honestly, the entire time I was just wishing I was playing Sonic Unleashed.
I found everything to be really drab and dull - while the graphics are high quality the actual scenery in my opinion lacked anything to make it stand out from other games.
I can't think of anything inherently bad about it, but i really honestly just wasn't enjoying myself legitimately for the first time since I played Sonic 06. I'm not really sure why, but after 14 stages i've just given up on Sonic Forces.