Sieg and Nightmare got beefed up alot in SC4, and they are far from being button mashers, I tell you this, they are very solid in SC4 compared to their SC3 counterparts.
Also, for me, being a Mitsu player, he has always been strong/very solid, but never overpowered, probably the closest that came to overpowered was SC2 Mitsu and even he wasn't #1 in SC2, however his KB2 was +6 Frame Advantage on block...which made it umm...insane!
If you want overpowered in SC, then that title belongs to Xianghua, she was ALWAYS Top-Tier in every SC game that she has appeared in, not to mention, she was always in the Top 3, and usually #1 at that. SC1, She was Top, SC2 she was Top, SC3 she was Top, basically making her the Jin Kazama of SC.
Very fast attacks, but most importantly, she was very safe off them, in a game where punishing opponents off of whiffs, and what not was a huge priority, her safeness benefited her greatly. She was also one of the few characters with a very good low, not to mention, she the most powerful 8-Way Running game in SC. This made X consistently good and beast in every situation, the fact that she didn't have to change her gameplay much from fighting other opponents is what makes her so good.
However, just like what happened to Jin in Tekken 5, here in SC4, Xianghua got nerfed quite a bit, so her reign as Top-Tier pretty much stops here.
However, Mitsu in SC4 although toned down from his other versions, he is still very strong in here, definitely a contender for being ranked in Top Tier at this very moment (obviously it can change) however, his 2KB is very strong, still has solid mids, but his 2k isn't as good anymore.
Sophie and Cass or should I say, the Sisters in SC4 look to be strong again, and yes, they look Top-Tier potential once more. So far, SC4 doesn't look too bad, we'll see if it can last longer then SC3 did at the tournament lvl, considering that game died in not a year, but just months.