This may be redundant with my above comments, but I blame the system. If Fantasy Hero had supplements like GURPS, I doubt I'd even consider D&D. Actually, if the Hero System, in general, had supplements at the GURPS level of quality, I'm not sure if I'd consider any other system.
Actually... I blame gamers who make judgements on a system without actually playing it. I'll be the first one to admit that I too first looked at the GURPS books and thought holy [----]! Then, I actually sat down and played the game and found it to be far less complicated than I had thought it would be. It sounds crazy, but there are times when I'd actually make the argument that GURPS is less complicated than D&D. Note: I'm not saying you fall into the category I mentioned; I simply feel there are stereotypes about the system which are not even remotely true.
For me, the other thing that I liked -and why I started buying the game- is because things made sense to me. I found that I could be running the game and make a ruling on the fly based on how I felt something should work, and that usually turned out to be right.
How I came to learn GURPS was I got to a point with D&D 4E* where I was unhappy. As I thought upon some of my issues, I realized that I wanted to try something different; another d20 game or another edition of D&D wouldn't be different enough. I decided I would take the leap to try something outside of the d20 system. So it was that I visited the local gaming store.
I browsed over the different options. My first thought was Rifts. I had a very brief amount of experience with it; it was a great game, but looking through the books reminded me of some of the problems I had with it. I eventually came to a point where I was comparing HERO system (I'm not sure of the edition) with GURPS 4th Edition. For some reason, when I looked through the GURPS book that was sitting there -and I mean looked through it with an actually attempt to understand the game- everything seemed to click. I cannot say what it was, but something about the presentation of the material and the layout hit a sweet spot in my brain, and everything just came together. Thus, I starting buying GURPS.
I did not completely forgo what I had learned from D&D though. Some of the lessons I learned from D&D helped me. From D&D 4E I took the idea that PCs and non-PCs need not always be built the same way. Did I really need** to know every single detail about Red-shirt Guard #32? No; I probably don't. GURPS doesn't tell you that you need to anyway, but a trap that some newcomers fall into is in thinking you need to know the point values for everything for everyone.
If a NPC becomes more important for some reason, I'll flesh them out later and make notes for myself. If a NPC is a major character such as the BBEG of the campaign, then -yes- I do sit and make him with more detail. However; generally speaking, I can fit several NPCs on an index card. Now that I have more experience with the game, I could honestly run some things from my head.
*This is not meant to bash the game. I'm simply telling the story. In an odd way, GURPS helped me to enjoy 4E. It did so by teaching me that not every game is well suited to every style of play. When I learned that, I felt that I was more able to relax and accept 4E for what it was. When I wanted something else, I'd play something else. Previously, the majority of my knowledge was D&D, so I kept trying to beat my head against the wall to get a game to work in a way different from how it was designed. I learned to think outside the red box.
**Though, I'm the type of DM who enjoys world building. So, if I have time, I'll sit and sketch out that information.