I feel a mini-article coming on, so here I go:
It used to be that a projector manufacturer would say 1,000 hours was the "expected lamp life". In reality, you got about 450 hours that were good, but the drop-off in brightness was significant, and they rarely lasted the rated 750 hours. Typically, these were the old 575-Watt Metal Halide lamps that had this problem. Also, the max brightness was between 500 – 750 lumen.
Then the 180 watt Metal Halide lamps were running much lower brightness – about 150 – 300 lumen, but would get an astounding rated 2,000 hours. In all reality, they really only lasted about 1,500 hours, but also had the problem of having serious color shift issues after the first 75% of the lamp life and a dip to up to 50% loss in brightness.
Next came the 1,500 hour Metal Halides that would burn much, much brighter – up to 5000 lumen, but were thousands of dollars a piece.
More recently, the UHP lamps hit the scene with 2,000 hours and very little color shift over the life of the lamp. These are produced in a number of variations from different manufacturers, but all have a few quirks (you didn't think they were perfect, did you?).
If you've been looking at projectors for a while, and been primarily using them to display video, you'd notice right off that these new ultra-efficient lamps were, well, blue. As in, the whites were really more of a very light shade of blue. Those old pesky Metal Halides, for all of their issues, really made video look much, much better.