AbdulAlhazred
Legend
So, let's guesstimate for a moment that I've done, say, 500 sessions of 4e, with 3 combats per session. So, out of those 1500 combats, I'm willing to say that no more than 100 of them would slow have made a crucial difference (defender is forced to use ranged weapons and can't defend, leader can't get into range to buff or heal, etc), beyond forcing someone to Run for one round or possibly not get an attack at all for one round, which would have had scattered appearances (mostly depending on terrain, initiative and enemy choice) in, say, another 200 combats - which, btw, is a benefit closer to Blind or Daze at that point, so still not bad.
The trick is that if Slowed does last for the encounter and you can count on doing it, you're further encouraging Ranged, and you're further encouraging parties that can exploit it. So, your party shifts to becoming more ranged and picks up a couple slowed and starts beginning combats 20 squares away. Now the Brute requires 3 rounds of full out Running just to reach the enemy, which means the combat is over before he gets to roll an attack roll (and miss, poor guy).
In quite recent memory, some examples from published adventures where Slowed would have been a serious problem (as in, not being able to do melee attacks that combat) if inflicted on my character:
2 Underdark fights where melee had to move 10-20 squares, some over difficult terrain, or jumping gaps to engage each other from first sighting
1 fight over floating earth motes that required jumping from platform to platform while being ranged attacked on the way in - this combat actually had traps that slowed, and they entirely changed the combat even though thankfully I avoided them for the first 12 or so squares of movement.
1 fight out in a Jungle where the enemy drakes and harpies were 10+ squares away to start and had a 40 ft pit to drop people into (Ugh, being slowed at the bottom of a 40 ft pit = 4-8 rounds of climbing! And can't even make ranged attacks from down there)
1 fight in a huge tomb where we had to get past a gauntlet of enemies and get under a sliding door to disarm the trap and save the day (get the treasure) in just a few rounds
EAST1-6, IMPI1-6, and SPEC2-1 P2 from LFR so not like just fights my DM was making up.
If I make the party all ranged, Slowed becomes far more useful since it's very very common for fights to start more than 4 squares apart - and such a party often has ways of controlling some portions of the battlefield further. Perhaps upping it to a crucial difference - if turned into a whole encounter penalty (removing a monster from consideration of combat, drastically altering the tactics of the combat) on 500-1000 of 1500 combats. I imagine in a lot of cases you'd see a reaction from the DM/adventure design where suddenly melee was always much closer, and the terrain was less choked or difficult, so that slowed would be less useful.
Mmmm, yeah, that's probably not radically different from what I've seen, but I'd have to say that your average party is still not having a lot of problems with being slowed. It definitely will come up some, but I don't think the game would change a huge amount if the condition didn't exist.
Were I intent on redesigning 4e to have less conditions I guess I'd just eliminate immobilized (restrained will do basically the same thing), slowed, and I'd make weakened something like a to-hit penalty perhaps.
I don't personally have a problem with marking mechanics. Yes, they're ubiquitous but the very ubiquity kind of makes them reasonably easy to deal with. You KNOW that whatever the fighter was just engaging is marked. Heck we don't even generally bother to track it, though with multiple defenders it could easily get a bit more complex.
The core of the issue is just the sheer number of little minor variations on very similar effects. I think if I were starting with a blank slate I'd actually consider adding a couple of conditions to reflect the most common effects. I'd strongly consider regularizing them all to have durations ending on the end of the turn of the target (with save ends or encounter long being more rare options). Then I would simply NOT design very many powers doing 'fiddly' things. Make most of them daily, make the maximum possible number of them instantaneous, and try to use one or another standard condition as often as humanly possible. I think this aspect of the game could have been made 2-3 times simpler without measurably impacting tactical depth. Some other things could also have been done to compensate and present additional tactical challenges, like slightly increasing the effects of terrain in general.
I'm tempted to delve into a bit deeper reflection on how the various parts of the 4e design ended up fitting together that bear on this whole subject, but I won't. Its a good design, just like anything when you go back and look at it afterwards hindsight usually shows some potential improvements. Its still one of the best designed RPGs around today overall.