There's a few things I miss in TF2 from TFC.
- Nades (both normal + conc)
- Spy feign death
- The Well design
- Demoman's demopack
Apart from the things listed above TF2 is an improvement, and you'd hope it would be anyway
Oh, and I'm unsure as to whether I like the new sniper skin as much as the old one (camo looks awesome).