Under the old system, a large part of character planning involved trying to get as many levels as possible in your key powers. The new system eliminates power levels entirely, and adds the concept of talents that can change how powers work depending on which talents you select. If that sounds familiar, that's probably because it's pretty much the same change we saw from Diablo II to Diablo III. In general I like it, especially since it means you see all of a character's powers (except the ultimate) by level 30.
The change to eliminate power levels had a big impact on items as well, since adding power levels was a big part of gearing character in the old system. Almost every item got a revamp, and for the most part you can do pretty well just by getting the unique items specific to your character plus a few artifacts. When I logged in some of my old characters, I was mostly able to jump right in without worrying too much about changing out their old gear, even though the stats on that gear had changed. I'm sure there's new ways to min-max stats, but the old stuff is mostly still usable.
Another major change is the simplification of the end-game character improvement system. The old OMEGA point system was extremely complex, so a key feature of the new Infinity system is simplicity. There are only 6 groups of 5 options for Infinity point placement, much fewer than the number of OMEGA nodes. I found it to be fairly easy to understand, and it certainly seems to have plenty of room to grow in the future if the developers decide to go that route.
I was less enthusiastic about the ramping up of difficulty. The "green" lowest level is still pretty much the same, but "red" and "cosmic" levels seem significantly harder than I remember. I suspect that the designers realized that power creep was becoming a problem - high-end characters with the best gear were just too powerful - so decided on an across-the-board difficulty increase during this major update. I'd rather that they'd paid more attention to the power creep to begin with. Still, at least this addresses the situation to some extent, and hopefully they've learned a lesson and won't make the same mistake going forward.
After spending a few days going back to some of my old characters, I felt fairly comfortable with the changes. It's a credit to the design and development teams that they were able to make these big changes in such a way that it was pretty easy for players to make the transition.
