Spikes vs. Meteors:
People seem very misinformed about spikes and I can't believe that its on people's lists of bad Melee mechanics.
1. In melee, an attack with a trajectory going perfectly downward can be meteor cancelled. However, some attacks (Fox and Falco's dair, parts of Marth and Falcon's dair, and Ganon's aerial down-b) hit opponents down at an angled trajectory. Since the meteor cancel mechanic doesn't apply to these trajectories, they cannot be cancelled. In Brawl, the range of downward trajectories which can be meteor cancelled was increased. However, iirc, the PMBR were able to add spikes back in by adding 360 degrees to an attack trajectory; creating attacks which could not be meteor cancelled.
2. The difference between spikes and meteors cannot be simplified to "one is useful" and "one sucks". As everyone knows, meteors and spikes on grounded opponents causes them to launch upwards with (I believe?) 20% less knockback but with normal hitstun. This is why Falcon's stomp>knee combo is so reliable because Falcon has copious amounts of frame advantage to work with before he lands the knee. However, another reason why Falcon's stomp knee combo works so well is BECAUSE it is a meteor. Since meteors have a vertical trajectory, they are significantly harder to DI, and, at the correct percents, lead to practically inescapable follow ups. Spikes, on the other hand, bounce opponents at angled trajectories which makes them unreliable vs. good DI.
tl;dr: meteors are better vs. grounded opponents, spikes are better vs. offstage opponents
3. Melee meteor cancelling is a much worse mechanic than spikes. In Melee, an airborne opponent can cancel out of a meteor with a jump or up-b in about 6 or 10 frames or something ridiculous. The fact that people complain about spikes but don't see the fault in this mechanic rendering all meteor smashes completely unreliable as a finisher is a complete mystery to me. Luckily, the PMBR recognized the fault in meteor cancelling and not spikes so this is the mechanic that got adjusted in PM; meteors are significantly more viable for an offstage attack while spikes are still kept to add more depth to characters and the game.