1. Unit variants that address a previous reliance on short range weapons by adding weapons like AC/10's or Large Lasers that SIGNIFICANTLY improve their range in TW, but don't make much of a difference in AS other than adding more heat or lessening the punch given by the number of tons invested. The Atlas RS is a massive improvement under TW, but under AS rules is actually worse in open field combat than the AS7-D it was created to improve.
Lots of Medium Lasers? Enjoy your improved Medium Range Band.
Lots of Large Lasers? Enjoy your weapons mattering a whole lot less than equivalent tonnage in Medium Lasers and Heat Sinks would have.
2. Zombie 'Mechs. Sure "ENE" means you're immune to ammo explosions, but those are a whole lot less likely in AS and you lose the secondary "anti-crit armor" from padding your 'Mech with boatloads of heat sinks. One of the reasons the Awesome is "less Awesome" in AS is that it doesn't have the insane crit resistance and ability to maintain firepower after taking structure hits that it does in TW. Same goes for the MAD-4A, WHM-6D, etc.
3. As someone else said already, Mechs that rely on the hole-punching ability of a very small number of very large weapons (Hollander, Hunchback, Kit Fox A etc.). In AS, they do decent overall damage, but completely lose the fight-ending ability to hit a soft spot.