For the Australian and UK audiences I'd blame the PAL speed up for that.
NTSC (USA) shows are speed up when converted to the PAL format. This usually means voices are higher than they are in reality and it effects low voices worse than others. Download an episode of Boston Legal in NTSC format and listen to how darn deep James Spader sounds.
All the PAL viewers of the world would be used to a higher sounding Optimus. The theatre has no such speed up issues and so he'd sound deeper to what we're used to.
Doesn't explain the Americans saying he sounds deeper though.
Maybe it is just age, but he's only just in his 50s. I suspect if anything it's a minor alteration of the voice after so much time not performing it.
It is instantly recognisable which is ultimately all that matters.
The movie is full of little references and jokes aimed directly at me (late 20s male who loved the original series and movie and who used the internet and computer games regularly since the 90s). Some really really obscure stuff and some darn in-your-face obvious stuff. I look forward to paying more attention when I see it again on the weekend. I'll probably talk about it more later after giving everyone a chance to see it.
There were moments when I literally put my head in my hands the dialog was so terrible, but other times I burst out laughing. Most frequently my jaw was on the floor admiring how beautiful it was to watch robots fighting.
Hearing Peter Cullen speak as the voice of Optimus Prime is almost a religious experience...
It's cheezy, it's harsh and the humans are an irritant. There is humour, a lot of it, some of which falls flat, you just have to go along with it. Just like the original.
As others have said, it's everything you'd expect from a Michael Bay movie, but in this context, I feel it worked.
Recommended!