Personally I like the combos a lot, it was always boring to me in WoW to essentially be spamming 1 1 1 1 1 1 1 1 a lot and ( in earlier expansions at least ) just sit around waiting auto-attacking for CD's and resources.

My main issue is that they're not engaging in some cases.
For example MNK is engaging because of the positionals and refreshing buffs/ dots, SAM is engaging since there's a certain level of freestyling you can do.
NIN isn't very engaging imo since it's just 123 and then 124 to refresh the speed buff.

Edit: Also SAM has buffs tied to two of the combo actions too.
I suppose that my general point is that the combos should be doing something different imo rather than simply being a higher damaging ability.