Can someone explain why a 64-bit system is a big deal?

If this was true you would be hearing a lot of noise in general as there are still a lot of 32 bit apps in use today.

what about the limited debuffs / dots on boss , is going to a 64 bit system can allow for more debuffs / dots on boss ?

It would allow them to increase that cap yes, but it wouldn't automatically do so.

I wouldn't think the two would be related at all. Nothing about 32 bit should limit the data structures you'd need for a debuff list.

It likely won't have any impact as from my understanding the debuff slots are limited by processing on the server and how the change would impact all npc's and not just raid bosses/npcs.

Eventually, windows will likely no longer support 32 bit programs. If EQ didn't upgrade to 64 bit, that would mean the death of it.