Overload can wreck shields and dish out a lot of damage against the Geth. The only reason you might want to hold back on upgrading fully is that Garrus should have this skill. If you plan on dragging him around as your soldier, then he can handle the overloads. You will need it at the second level to unlock incinerate.
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Incinerate tears through armor. It's incredibly useful against krogan and just about any other enemy with a shred of armor actually. Incinerate can also just cause a lot of damage to organic life. Being able to fire off a damaging ability from cover is incredibly handy. If you don't want to spend points on this Mordin can also use it. If you plan to keep him close, then you can let this slide a bit.
AI hacking is pretty cool for most of the game, since it can turn mechs and geth against each other while you watch from cover. There's just a certain joy from sitting back and watching your enemies kill each other. AI hacking is a bit lacking on harder difficulties. You can only use it once the shields are down, which limits the time that you can use it in combat. It's also a little harder to just stop firing once you've taken their shields down. Another downside is that upgrading it only boosts the length and provides minor upgrades like giving the possessed bots shields and eventually offering an area attack at the end. It's certainly a fun ability for quickly shifting a battle, but it is going to be absolutely useless a surprising amount of the time. It also does nothing against the collectors, so keep that in mind.
Cyro Blast isn't that great. It's handy enough, but just freezing the enemies doesn't seem to do much good for a real battle. Since you will only deal with rushers occasionally, it isn't that useful. While it is fun to freeze the raging krogan or a wave of husks, you'll be better of spending the points on something else.
Finally the unique little Combat Drone is pretty cool and you'll want to upgrade it quickly. Enemies should always focus entirely on the little ball. Any enemy that you fight will stop shooting at you and focus on the ball. Throwing out a combat drone saved me several times, since it buys time to heal or move. It also lets you line up nice shots too while also doing a surprising amount of damage near the end. It can also flank enemies and it does a good job of flushing them out of cover so that your squad can gun them down easily. Best of all, it has an absurdly short recharge time. It's basically like having a fourth squad member.
