Throwing in my two kopeks...
The obvious answer is that there's nothing wrong with it, but as your amount of games you've played accumulate, your tastes and those of your group may change, and that's where the other d20 systems, and other games entirely, come in.
Those of us who've played 20+ game systems have either (A) been at this a long time or (B) have enough free time to try these systems out, having run through D&D likely and either tired out of it a short while or jsut plain found it wanting.
Other d20 games have every useful features, in some cases offering solid rules alternatives for "plugging holes" in D&D. For instance, when 3.0 first came out, there were no guidelines for underwater combat - several fans and d20 companies jumped on this idea. There STILL are not a good solid set of chase rules for D&D - chases just aren't a big part of basic play, so you can either wing it, which a lot do, or go to a system like Spycraft and perform the Sincerest Form of Flattery on the game system.
As for transition between d20 systems, let me say that there are people in our group who dislike playing non-d20; to pick up most other d20 games and settings, all you need to know is (1) new skills, (2) new classes and races, (3) new feats, and (4) any new rules that diverge strongly from D&D. THe basic mechanics are there, there's no funkiness to new rules
("to throw an object, take the square root of pi, multiply it by your age differential on Table 3-1, and add the average of your height multiplied by your weight...") Metaphorically speaking, the thing's still a car, and the gearbox/steering column/seatbelts are all in the same place, they're just different manufacturers.
Half of my players are gung-ho about any new system, but the other half are hesitant any time I pull out a non-d20 system, because they feel the learning curve for them is a waste of time, when they could be spending that time gaming.
Steve Jackson of GURPS had the right idea -- he just didn't have the mindshare that D&D has to make GURPS the dominant universal system. (Some would say he didn't have the right ruleset, but that's more a matter of opinion.)