I think the combos heavily hinge on the TP factor.

Not only will a successful combo grant you the bonus indicated; TP cost will also be waived for the weaponskill used to execute it. For a combo to be valid, however, the weaponskill in question must be used with the class or job to which it belongs.
Has been interpreted as your opening weapon skill costs TP. Your combo weapon skill does not cost any TP and can be used immediately. But I don't see this being the case. Otherwise, a 1500 TP move chains into a powerful two to three hit combo. At 3000 TP, one could execute two combos in quick succession, which would certainly make battles more fast paced in many senses.

Others have interpreted as, your opening weapon skill costs TP. You need to rebuild TP to use the combo weapon skill. After execution, you retain the TP.

And we're wondering about the time limit between combos, especially now that there's reduced TP generation from auto-attacks and overall higher TP costs. I'm thinking there will be an indefinite amount of time between weapon skills as long as the mob is alive ~ engaged.

My thoughts on the combos are that some folks are gonna try them out, see that some of the effects aren't quite worth it, and sometimes go for that "finisher" move first to end the battle instead of everyone trying to go for their individual combos.