There's a lot to that question, but the basics are that more than 4 cores don't mean anything for gaming, the 8150 doesn't perform like a true 8 core CPU should (8 integer cores, but only 4 Execution units), and the Bulldozer architecture is much slower than Intel's Sandy or Ivy Bridge (IPC, like. Z1NONLY said).
You also can't compare speeds between them because the architectures are so different (and of course, they're from different manufacturers).
Main overall point: Don't be fooled by AMD's "more cores is best for EVERYTHING" argument and their claim that the 8150 is a true 8 core CPU. Very little main stream use for more than 4 cores, anyway (just in video encoding, mostly).
Games don't make use of more than 2 or 3 cores so you could have a 16 core monster and it won't make a difference untill the games use that many cores. So it comes down to what those two or three cores do when compared against each other and the archatecture and instruction sets are what makes the difference as ZINONLY said and once it's pattened another manufacturer can't copy it or use it without permission from the pattend holder and Intel isn't goung to allow its competitor to use it. So it's always a race to be the first.
If you have some process that requires 8 cores then the 8150 is the cpu for you or you can get a six core Intel cpu with hyperthreading and have 12 threaded cores.
Most of this has been covered but I'll repeat it in my own words.
IPC(Instructions Per Clock) is much better on Intel processors, heck even the old Phenom II has higher IPC than FX. This is caused by a large number of differences in processor architecture.
With games, very few can properly use 4 cores, beyond that the cores just aren't used except in a select few games. The FX 4100 and FX 8150 are essentially the exact same chip in the realm of game performance because the games won't use the additional cores. I5 walks all over the FX 4100 in games, so of course it'll do the same to an 8150.