My thoughts on any level caps has always boiled down to a few factors. How quickly will I reach the level cap? In most every RPG I've ever played with low level caps (including some of the Mass Effect games), I'd reach the level cap before finishing even half of the game. Mass Effect 1, I reached my level cap at a good place... My second playthrough. Of course, it helped that you were capped on your first playthrough and then given your last ten levels on the second playthrough. This at least made the second playthrough worth doing. Mass Effect 2, I reached my level cap BEFORE THE MIDDLE OF THE GAME. Yeah, I got my 5 levels bonus from carrying my character forward, but then topped out before the middle of the game. I hadn't even done that many sidequests either. I later started up a file without carrying a character over, and found the same issue. I would get a little farther into the game, but not much farther than halfway and be maxed out again. It feels silly to be maxed out before exploring a majority of the content to me. Then, we had Mass Effect 3. I imported my file again, got 30 free levels from it, and then proceeded to max out about a quarter of the way through the game again. I've also had this issue with Fallout 3 and Fallout New Vegas. It's very easy to hit level cap without even trying, and then it just feels like there's no reason to care about doing Quests anymore. Combat loses its flavor when nothing is gained from it (no xp, no cash) as in Mass Effect 2 and 3... Quests lose their flavor when you can't gain xp from them, and only gain the same amount of loot you'd get from just random encounters or searching treasure chests instead.
I find that I enjoy high level caps that I will rarely ever reach more than low level caps that are extremely easy to reach. Unfortunately... Those are the ONLY two varieties level caps come in. Either you max out too quickly, or don't max out at all. I'm not an advocate for gaining max skills in a game and being all-powerful or anything like that... I just believe in GREAT game design. Unfortunately for most developers, that doesn't include anything they do. If you include a system in your game, ANY system in your game, it needs to remain useful and relevant until the END of that game. If you have a system in which you gain levels and choose skills after those levels, it needs to remain functional until you beat the game. If it tops out long before the end of the game (as most low level caps do), it's not a useful feature. It could be better implemented as a system in which simply completing Quests or training skills would give you these necessary boosts or skills for your character (or characters). There is literally no reason for a "level up" system to exist if you're going to max out the character by mid-game or by 75% game completion mark. The functions of a level up system in a low level cap that maxes out quickly are better served in OTHER ways that would regulate your power (as is the point of a low level system in the first place). You could more easily handle it by "Do a quest, pick a power", or "do a quest, level up a power" if you wanted to implement a "low level" cap.
Now, if we're talking about a game where you have 20 levels and you reach level 20 about 2 or 3 hours before finishing the game, I have no problem with that. A low level cap in that case wouldn't be out-of-place or even a bad thing.
HOWEVER! That is not how game programmers do it for low level caps, and here's why: It's much easier to program 25-30 monsters in a 1-20 level range in order to add diversity to combat than it is to program 200-400 monsters in a 1-99 level range in order to make combat worthwhile and keep xp values flowing in good chunks to reasonably obtain level 99 if you wanted to. Honestly, that's the trick right there. Most game devs don't talk about it, but that's what they're doing. A low level cap is basically just a way for game developers to save time, money, and resources on the enemy assets. D&D is even guilty of this to an extent. All they do is give you something like 10 monsters per level, and then you buy expansions for another 10 monsters per level. With a cap of 20 to 30, this becomes fairly easy for them to do, since they don't really have to worry about balancing. Balancing is an issue for a GM to handle, not the designers. Honestly, creating a creature asset for D&D takes no longer than 3 minutes and in the latest versions they even tell you how, and give you the option of rolling dice on these monsters to determine what they are, if you have no idea what kind of monster you want. It's so easy for D&D to make new creatures that they can have the DM do it by rolling D20s. But, if we're talking video games... If you have low level caps, you don't end up fighting very many enemies. There will be a few species of enemies and then they'll have "scaled to your level" equipment and stats with new names (so that they SEEM new, despite just being buffed creatures with different equipment), and you'll fight them ad infinitum nauseum until the end of the game.
So, there you go, that's the trick. Low level caps = few enemies. High level caps = more enemies. Just look at all the enemies in any standard Final Fantasy game, even the ones that are just palette swaps that have different abilities or buffed stats. Still more variety than the something like 7 total enemies with their level specific types in games like Fallout. Sure, there's like 10 different Super Mutants, but they're all a Super Mutant. Sure, there's like 50 different people, but they're all just people with different equipment. Sure, there's like 6 kinds of ghouls, but they are still just ghouls who run up and melee you, with only "Glowing Ones" doing anything different than the rest. If the level cap were higher in games like Fallout, the devs would ultimately have to design more monsters or at least higher leveled monsters just to cope with your now current level in order to keep a steady flow of xp into your character.
Basically... in short... Low level caps are a great way for a sneaky dev to be lazy without most players knowing that's what they're doing.