For GM's Day I treated myself to a copy of 1e Top Secret, the old TSR modern espionage game. It was fun reading through the rule book - brought back lots of great memories from gaming during my high school years.
I was curious to see if TS would hold up as a modern gaming system compared to d20 Modern - could I run the same kinds of games using TS as I do now? After spending a pleasant evening re-familiarizing myself with the system, I realized how klunky some of the rules are compared to d20 mechanics - combat that's resolved literally second-by-second, pages and pages of charts for handling different forms of martial arts in hand-to-hand combat, skill and combat modifiers that range from -5 to -100 or more (chance is resolved using percentile dice). While I still like TS's char-gen system (secondary and tertiary attributes rawk) and "areas of knowledge" (skill system), only the former has anything on d20 mechanics.
When I started gaming again a few years ago, I was pleased with many of the changes in D&D and the d20 mechanic generally. It's an elegant system for resolving chance in the game, clean and consistent (at least to the extent that one keeps close to core - the eventual downfall of 3.x and their progeny will be the proliferation of rules in the forms of feats, class abilities, and new actions). Never experiencing 2e, I relied on my "1e knowledge" and found that I could create game-worlds and run adventures like those I did when I was younger: crawling through lightless dungeons, trekking through howling wilderness, sneaking through city alleys and across rooftops. The 3e mechanics meshed well with my 1e gamemastering.
That's not to say that there aren't things I don't care for in the current incarnation of D&D. Some are holdovers from 1e: the Vancian magic system has always bugged me and I've learned to tolerate but never love hit points. Others are more specific to the current edition: CR, the proliferation of magic items, "game balance." None of these limitations prevent me from running the games I want to play, however. When I make houserules they're generally small, niggling things that I modify to achieve a certain feel, or setting conceits such as the prevalence and availability of magic in a fantasy world.
Overall I consider myself content with the system - it works for me.