SP Levels: Quite good. Lengthy, challenging, detailed, and creative.
Character WADding: Horrendous. I think that removing customization of abilities entirely was just about the worst thing that could have been done. I can see removing slingring, since it goes against the style of Sonic a bit, but I still think that was a cool option to have. Either more abilities should be added, or custom abilities and/or slingring should be re-added. Otherwise, there will be no such thing as a unique character WAD. If multiplayer is an issue, you might consider making slingring/custom-ability mods banned on the master server.
Level WADding: Enough to counteract my negative feelings about the above. The ability to put multiple effects on a single makes making a unique level much easier. There are still a few things I would still want in (like Once Per Character or an easier No More Enemies), but for the most part, with the new effects, new textures, new objects, and multiple effects per sector, the engine is now easier and more powerful than I have ever seen it before, and I have a much better time creating a truly unique experience for the player. However, I do wish you had kept difficulty levels, but I can see why you left it out, and it is quite an acceptable loss.
SOCcing: I like the new actions quite a bit. I don’t think I used any yet, but it is quite good to know that I have much more power in custom frames than I did before. The SOCEdit.exe packaged with the game is powerful, but it still has a few minor glitches that need addressing, such as Var2 on the frame editor and the .cfg selection on the global variables.
Saving: Automation is good, but you might still have a kink or two to polish off. All in all, though, an improvement.
Devmode: Now looks less messy than before, but for those who are making EXE modifications, you might consider making a variable that will show the more detailed information.
All in all, the only negative I can see thus far is the changes to character wadding.