I replayed through it earlier this week in preparation of Reach.
Basically, yes; it hasn't exactly aged that well when compared with current FPS games. That's from a technical and game design perspective only though.
It was clearly the first in what was intended as a series from inception so there were quite a few things that were likely intentially omitted in the interest of upping the ante in future sequels.
The main type of person who would still find great entertainment value in the game is likely someone emotionally invested in the Halo series/universe. If you're not in that category, it's pretty understandable if you're inclined to wonder what all the fuss is about.
Offhand I'd say that two of the main aspects of the game that were responsible for its success were the epic scale of the setting and universe of Halo (it's easy to underestimate the effect a well conceived back story can have on a game) as well as the local multiplayer, the likes of which hadn't really been seen since Golden Eye.
There was a pretty huge hole in the console market for a well made FPS game (with good local multiplayer) at the time Halo was released. Bungie had significant prior experience as an FPS developer (Marathon Trilogy developed for Mac OS) making them well suited for the task.
