One surprise that I ran into way back was when I first played a Hermes II "Mercury" variant. The standard HER-2S isn't terrible as a Medium command element of a Light recon lance, but doesn't have the firepower to engage anything serious, so I wasn't expecting much out of yet another Hermes, especially with no long-ranged gun, just a few MLs, MGs, and a Flamer. It quickly became apparent that the 7/11 speed gave it enough mobility to put the weapons where they were needed, the 11 heatsinks were sufficient to fire everything except either the Flamer or one ML turn after turn without building heat, the armor was quite respectable for its weight class, and the combination of MLs followed by MGs allowed it to tear up back armor and then fill the holes with tasty bits of lead (crit-seeking), scoring a couple of surprisingly easy kills against 'Mechs far above its weight. I'm still a big fan.
The MAD-3R was an early favorite, and I still love the old artwork for it, but the unpadded ammo in the left torso, combined with the relatively thin side torso armor (compared to just about every other location) has plagued it in several games, and it's definitely light on heat sinks. There are better variants, or else a simple change or two to deal with the ammo problem, like swapping the AC for a LL and some extra heatsinks, or to an LRM-10 rack, a couple of heatsinks in the side torso (to provide some padding for the ammo), and one more ton of armor mostly used to beef up the sides a bit, could have fixed it. It's GOOD, but so close to being "RIGHT" without actually being there that it hurts. Of course, much of that was due to the design rules change between the ORIGINAL rules and all of the later versions, which moved most of the heat sinks to inside the engine itself (originally, ALL of the heat sinks in a 'Mech took up slots), so the stacks of heat sinks protecting the ammo were no longer there.....but still not as "splody" as a Crusader (aka "Roman Candle") or a Trebuchet. I still have a nostalgic affinity for the venerable MAD-3R, and enjoy running them, but am well aware of its issues.
The Whitworth was another surprise when I first used one, especially as it was frequently derided on the boards as the "Worthless". Turns out, it's a fantastic "bracket fire" design: either you fire the long-ranged weapons, or the short-ranged, but not both. It's got as much armor as a Dervish that's 15 tons heavier, packs roughly the same firepower, and surrenders one point of movement and jump distance for those 15 tons. Not a bad deal. Since I was playing a mercenary campaign at the time, heavily constrained by prices, the smaller engine and gyro on the Whitworth made it an affordable option which I had some initial hesitation over. Once I fielded the 'Mech, it contributed effective fire support to the rest of the group (from a range where speed doesn't matter that much), and when the ammo ran out, its triple MLs and respectable armor were surprisingly useful against the beat-up front-line units still remaining on the opposing side. The Catapult serves the same function: providing effective fire support from well behind the front line until the ammo runs out, and then you discover that the REAL threat is the MLs.
