No, they shouldn't be pretty much identical. That's comparable to suggesting that all car models should be pretty much identical because they have tires.
There is a great variety in motherboards, most of which you shouldn't concern yourself with. Purchase your new motherboard based on chipset. If you do that, you're almost guaranteed to get any features/expansion slots that you need.
There really isn't a "beginners" board out there that I'm aware of. Really, there isn't much to assembling a PC. If you can build a castle with Legos or Mega Blocks, I can almost guarantee that you'd be able to assemble a PC. It's slightly more complicated in the sense that there is more to know and more risk involved, but, if you take your time and cross check the moves you're making you can't really mess up.
The people that fail at assembling their PCs are those whom are impatient and dive in hoping for the best, or, are stubborn and think they can pull it off without any assistance.
What is your budget for this new system and what are you looking to do with it?
