1) Money. The game could cost anywhere from $40 to $60 (30 to 45 Euro) and even though ARMA is my favorite game for many reasons, I'm not jumping to drop that much dosh on a new release. Especially considering how buggy ARMA 2 is even after this much time (not as bad but still noticeable).
2) Bugs and polish. ARMA 3 will be buggy as sure as the planets turn. This is to be expected considering how massive an undertaking it is. So, waiting a few months for the massive bugs to be removed would be best.
3) Missions. We need missions and mods and such to be up to date before we should make the jump. Mods aren't as important but client side mods do make a difference. Not that that's a deal breaker. Missions are a definite because we'll be needing our main missions (Cholo and the lot) to be ported.
4) I AIN'T GO NO MONEY JUST WAIT 'TIL SALES AND MAYBE THE GOTY EDITION OR AN EXPANSION SERIOUSLY
