Rolling for damage, here. The fixed damage for minions in 4e is something which really irks me. From a DM point of view it makes combats too predictable, removing an element of suspense.
It only removes an element of suspense in a meaningful sense where it was a meaningful element of suspense, though, not to be cute about it.
In most cases, the major element of suspense is hit/miss, in D&D. It's often 50/50 or close to that, and typically a miss means zero damage. That's your very major element of suspense.
Damage is typically either:
A) On a narrow range - like with a small die, or a larger die with a relatively large bonus. In either case it's not very suspenseful. (Examples - 1d6, or 1d10+9)
or
B) With a very strong average - i.e. lots of dice being rolled, particularly smaller dice. (Example: 6d4 or 8d6 or the like).
With B you might get some "fake suspense" by rolling the dice, because most players don't really instinctively get averages, but even then, it's questionable whether it's worth the time, especially with smarter players. D4s in particular are annoying enough to handle, and with such a tendency towards average results that if a game tells me I don't have to roll them, I will not roll them.
You only get
strong suspense from damage rolls (as opposed to to-hit rolls) if:
C) The damage roll is a fairly wide range, and the PC(s) have fairly few HP - for example, if the PCs all have 4-12 HP, and the enemies are rolling a damage with a wide range - for example - 1d12, or perhaps 2d10 (even the latter is average enough to reduce the suspense considerably for "wise" players, though).
Once PCs get more HP, or if enemies roll more dice, or rely more on damage bonuses than dice, rolling those dice becomes way less relevant.
I'd probably roll all dice for non-swarms from level 1-4 or so (assuming 5E sticks with it's current "low" HP totals for low-level characters, rather than reverting to playtest 2's much higher values), but I think after that, average damage would be a good idea for most mooks.
Also, re: 4E minions, I don't think it's a tenable position to maintain that it actually makes combats predictable, as to get the same damage values, you'd almost always be rolling multiple dice with a bonus - for example, the minions in my last level 13 combat in 4E did 12 damage - that'd probably be 2d8+3 - which is going to average so close to 12 over, say, ten hits, that it will not make combat significantly more predictable. The hit/miss is what makes D&D combat unpredictable and swingy, not the damage values - certainly above low levels. This is easy to demonstrate mathematically, too, it's not really something one can argue about much. Only large single dice and small HP totals make for strongly suspenseful damage-rolls.
