World of L_Tiene has covered most of this in her answer, but I thought it might be worth adding in my perspective.
GURPS isn't the only roleplaying system that requires you to roll under a certain number, it's just one of the more well known ones. Warhammer and it's sub-systems also do that, and I'm playing in a game system called Flashing Blades that does the same, although Flashing Blades is a little known RPG.
From my own perspective of learning GURPS in the last 3-4 weeks, I didn't find it difficult to pick up. Part of the ease of the GURPS system is that by comparison in R≥S systems, the GM sets a TN, applies modifiers and then you roll (or mods are applied to the roll, it makes little difference), all of which requires having some sort of comparable scale between different sorts of tasks (easy, medium, difficult, etc.) This can cause some disputes and requires a delay while you ask questions.
With R≤S systems, you already have the TN given to you: it's the stat/skill you've got on your character sheet, so you already have a fair idea of your odds of success without having to ask any questions from the GM. In my GURPS game, I know I can make a roll and have some idea whether or not I might have succeeded before mods are applied.
R≤S also allow skills to rise as high as you have points to spend in them. In Warhammer they use a d% and you can get to 100% and even beyond. That is, there really is no cap on how high I can take a skill/stat except in some cases for it being prohibitively more expensive. With R≥S systems there is (ultimately) a desire to have the
skill stat as low as possible
|Yes, I know this isn't strictly speaking how most R≥S systems work because you're still trying to get skills as high as possible, but it's the relative difference between what you add to a dice roll compared to the TN that I'm talking about |
relative to the TN to make it easier achieve the desired outcome.