Restricting characters to 16's has the effect of heavily discouraging (or punishing) certain choices of races or builds.
Random rolls has the potential to prevent a player from being able to play the type or build of character they wanted to play because they weren't able to roll the right stat distribution to meet the requirements for it.
My advice is just use the default rules. You say you are used to 3E rules and not 4E rules, and it's a good idea to actually play the 4E rules before trying to tweak them.
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This answer is one reason I came here before I even have gotten my group together. I needed this type of input.
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That's also ignoring the impact on various skills. The simple truth is, what you see as a too optimized character has to sacrifice considerably in order to get a high score. It balances itself out fine as is, and doesn't require any heavy-handed DM restrictions on what type of players they are allowed to play.
Unnecessary house rules that cut down on player options can be really frustrating to players.