Dragontamer: Most Mega-Mawile are running enough speed to make what you're saying about defensive pokemon like blissey, and slower pokemon like machamp a non-issue.
As for Medicham vs. Mawile: I think the main difference, and why I prefer Mawile (and also what ends up being what makes me think Medicham is not a top tier mega) is typing and bulk. Mega Mawile is bulky, and it's that awesome typing that helps it. Both cause switches, and have the power/moves to cause the pain, but Medicham is so much harder to bring in than mawile. Being faster is nice, but Medicham is not that fast, not fast enough for the lack in bulk it trades for it. Medicham isn't fast enough to sweep teams leaning towards hyper effense, and isn't bulky enough to switch in often on bulkier balanced teams. Seriously, this thing cannot take a hit to save its life, and being swept off its feet by almost every common priority user doesn't help one bit.
Of course this thing is dangerous, I mean look at that attack stat, but I think it's outclassed in the current meta by pokemon that can cause just as much damage but don't need as much help doing so.
But one thing Medicham has over Mawile is coverage. IMO Mawile just doesn't have the coverage to handle most defensive cores.
Mawile requires Sucker Punch or Substitute to not get revenge killed. That's one slot gone. Swords Dance is another, or else it can't reliably OHKO many revenge killers with Sucker Punch (but I wouldn't call it 100% necessary) and can get walled more easily.
So, assuming three available slots for damaging moves that aren't Sucker Punch (two if you have Swords Dance), you have to make choices on what coverage moves you want to run.
Without your STABs in Iron Head and/or Play Rough, your damage won't be as good as it could be.
Without Brick Break, you get countered by Heatran, who can OHKO with Lava Plume.
Without Fire Fang, you get walled by Skarmory, who can phaze you out and make you lose any boosts you might have had.
Without Iron Head, you can't 2HKO specially defensive Venusaur, who can use Sleep Powder and/or deal a lot of damage with Earth Quake.
Without Play Rough, you miss out on your best STAB, and are countered by bulky Gyarados and walled by Mandibuzz.
Add in even more defensive Pokemon that have different resistances/weaknesses, and it gets even harder to not get checked/countered by many defensive Pokemon: especially when you don't outspeed them.
On the other hand, Medicham, with a moveset consisting of HJK, Psycho Cut/Zen Headbutt, Thunder Punch and Ice Punch, can single-handedly dismantle any defensive core that doesn't include Sableye, Aegislash or odd stuff like Dusclops and Cresselia; and Medicham can handle Aegislash by running Fire Punch (but then gets walled by Slowbro) or all ghosts with Foresight (I don't recommend it though.)
But, most importantly, Medicham IS a lot faster. 100 speed is not very good for a sweeper, but it's fast enough to outspeed most defensive Pokemon. Mawile has extremely poor speed, and is outsped by Heatran, Venusaur, Rotom, Gliscor, bulky Gyarados, Skarmory, etc.
Tl;dr my opinion is that Mawile does better against Hyper Offense teams, having the bulk and typing to take several attacks and the priority to deter revenge killers, while Medicham is much better against balanced and defensive/stall teams, having a ridiculously strong attack in HJK and the coverage to handle anything that resists it AND enough speed to outspeed nearly all defensive Pokemon. Medicham can either obliterate a stall team with zero help from the rest of its team, or get rid of a defensive core on a balanced team which can allow the rest of Medicham's team to handle the rest. These are two things I just don't think Mawile can do as consistently.