I had a issue like that too this too - Pi would just sit there with LEDs on and not (continue) boot
The only I could get it to boot, was to unplug the HDMI cable, disconnect, reconnect power, let the Pi power up. Then reconnect the HDMI and do ctrl-atl-del to restart the system and get the video back. In a nutshell, it would only boot up with HDMI on a 'warm' boot.
Latest firmware as a of about a week ago (has the emmc clock fixes) does not have this problem ..
The other thing that you might want to check is having over-clocked or emmc clocks set in /boot/config.txt, just try commenting them out. Note that /boot is just a FAT32 partition and visible on on a desktop PC (Linux, windows ETC ... ) and also contains the kernel and firmware ...