There's good and bad for where you are now and for higher populated realm:
Where you are now, you have the opportunity to build a guild slowly and build a reputation easily.. if you're patient. With a lot less raiders around, you'll have to get in people with less experience and build them up.
Positive: opportunity to build a guild with a reputation that will attract raiders
Negative: less raiders available on realm with more patience needed to nurture raiders
With a higher pop realm you have the reverse:
Positive: more experienced raiders available without the need to train them
Negative: have to compete with bigger guilds that have a bigger raiding reputation
It's up to you which of the negatives you can deal with personally and as a guild.