13th Age has a short list of status effects, and I consider that a big plus for a tabletop RPG. One of my big pet peeves is when a game uses multiple different "flavors" of what is essentially the same effect (i.e. D&D 3rd Edition's Shaken, Frightened, Cowering, and Panicked; Fatigued and Exhausted; Dazed and Dazzled; Sickened and Nauseated; Paralyzed and Stunned; etc.). I much prefer a minimum amount of rules which serve to represent a general concept. The 13th Age "Dazed" condition does a good job of this, as it can stand in for any situation where you'd have trouble attacking (but aren't prevented from doing so altogether).
However, the list of Conditions is missing some functional concepts that many D&D players are used to. Up until EE6 there wasn't a single form of mobility denial, but thankfully "Stuck" made it into the game. Having anything in between "can't move at all" (Stuck) and "can move fully" is difficult in a system where movement is handled in abstract range bands, especially when granularity is so low that we have only "nearby" and "far away." That's not to say I don't like the simplicity of the range bands, because I do. It just means that we need to get creative to represent things like "slow," "difficult terrain," or "prone."
Quick, streamlined combats and combats where you can take tactical advantage of the terrain are not mutually exclusive. Sure, 13th Age advises a "don't sweat the modifiers" approach but it can be really difficult for some players to describe their character's action if they don't have any mechanical incentives to work with. Why try to put difficult terrain between you and an enemy if it has no effect in-game? More importantly, this is a case of the game rules not supporting the game fiction.
The following houserules can be incorporated as new Conditions, or simply as guidelines that a GM can use as rulings in case players decide to get creative.
Whether an enemy is hobbled by a weapon strike to the leg, slowed down by a spell (perhaps as a result of Vance's Polysyllabic Verbalizations), or is forced to move through terrain affected by another creature (i.e. an enemy triggers a rock slide, or a spellcaster magically animates the vegetation) this quick and dirty condition can be used.
If the slowed creature attempts to move it must roll a save. On a failure, it fails to reach its destination. Usually this is a normal save (11+), but depending on what is slowing the creature and how far they're attempting to move, the GM may rule that it's an Easy (6+) or a Hard (16+) save.
This is a variation of the "Slowed" Condition.
Sometimes it makes more narrative sense to allow a Slowed creature to make a skill check instead of a save. Generally this will be the case when the terrain is not being affected by another creature. This check will usually use DEX (icy terrain, rubble), STR (dense vegetation that needs to be muscled through), or WIS (if a good path can be found), though whatever ability makes most sense given the type of terrain or the player's approach to the problem should be used. The DC is determined by the Environment, as with other skill checks. On a failure, the slowed creature fails to reach its destination.
Quick, duck behind that barrel!
If you use a move action to get behind an item that grants a substantial amount of cover, ranged attacks are made against you as if the attacker were Dazed.
Characters can drop prone to avoid ranged fire, though it's not as effective as taking cover, or they may be knocked prone by powerful attacks.
You can drop prone as a quick action. Standing up can be done as part of a move action, but if you wants to move anywhere else you must succeed at a Normal Save (11+). While prone you are Vulnerable to melee attacks, but ranged attacks made against you take a -2 penalty.
*Note: GMs should be careful about introducing reliable ways for players to gain bonuses like this. For some, part of the fun of ranged combat is trying to find cover and eliminate the enemy's cover. However, it does have the potential to slow the game down. After all, the intent of the "don't sweat the modifiers" rule is to ensure that things run quickly and smoothly. Keep in mind that if the players can take advantage of the environment, then the monsters should too. Make sure that combats don't devolve into both sides hiding behind cover, hoping to get lucky with their debuffed attacks before the enemies do.