Well, consider the gameplay impact of attack and then move (in that order):
1. Assume the only way to move and then attack is to charge and make a basic attack. This increases the importance of having a good basic attack.
2. It becomes harder to avoid opportunity attacks. Spellcasters and ranged attackers now have to choose between casting a spell/making a ranged attack and provoking an opportunity attack or moving away as their action for the round. No more shifting into an unthreatened square and then carrying on regardless.
3. Cover and concealment become harder to negate. No more moving into a position where you're no longer blocked before attacking. You have to deal with whatever is the tactical position at the start of your turn.
4. Given the above, powers and abilities that allow you to move before attacking become more valuable. Similarly, powers and abilities that allow you to move an enemy adjacent to an ally or out from behind cover or concealment so that an ally can get a clear shot become more valuable.
5. It also becomes more useful to have a good ranged attack or a reach weapon, in case you have no adjacent enemies at the start of your round and you can't charge anyone.
All in all, it will be different, but I'm not sure it will be all bad.
Actually. I see Themes being the main method of gaining changes to action.
Lurkers can move then attack without charging.
Skirmishers can move then make 1 attack at any time before the end of the attack.
Slayer gain +2 t attack and damage rolls when charging.
Enemies cannot move after attacking a Guardian.
etc.. etc..