Metroid is a great classic. Those that didn't play it already though: Pick up Metroid: Zero Mission instead.
I'm marking this as a Old-School; If you can't tolerate an old school game then skip to the last paragraph otherwise, read on; I'll slam the negatives:
Navigation is Frustrating / No In-game map:
Those of you that grew up with everyone else doing your homework will probably be frustrated with no 'in game map'. Do yourself a favor and go to a game website and download a map and print it out. You can find 'just maps' that leave the exploration of power-ups up to you, or get it with the power-ups on there. Then you won't get lost. The original metroid world is far from huge; I'm amazed a reviewer for Games didn't think to look for a map if that was a gripe.
Flickering:
This is a NES port. And yes that's how the game played back then. I would say it would be nice for an emulator to solve this problem. It would make all sorts of games that sucked in their day good (Can we say Gradius III?). But it's far from problematic as at least it doesn't cause major slowdowns. And you can always try shooting the monsters then wouldn't be so many on the screen.
Super Metroid has made this obsolete:
Unless you have played the first Metroid most of the creepiness and awe of the Super Metroid would be lost. How can a sequel obsolete the story line of the original? It can't. If you really can't make it past these negatives do yourself a favor and pick up Metroid: Zero Mission (not Super Metroid) which is the Metroid story-line equivalent with those gripes 100% removed.