I am a new raid leader for a small guild starting 10 mans, 2.5 years ago we were using Deadly Boss Mods but I am sure much has changed since then. I know wow has an inherent boss mod, but is there something I should be using over the top to keep raid aware of certain abilities/timers?
DBM, BigWigs, or Deus Vox Encounters are still very much needed unless you want the Raiding experience to be a bit more "in the dark". Other than that I'd recommend a Spell/Cooldown tracker such as DBM's Spell/Cooldown tracker (needs to be downloaded separately btw).
Being able to see buffs/debuff on friendlies/enemies is also important so grab an addon that allows you to see that quickly. IIRC, Grid, Pitbull, and various other addons allow you to see this. Just pick the one you're most comfortable with.
Apart from these mods a mod for tracking loot can be useful as well, depending on what loot distribution system you use (CT Raid Tracker, etc.).
Then there's various stand-alone addons for specific boss fights (e.g. addons for Blood Queen Lanathel's bites, Professor Putricide's plague, and Atramedes sound debuff tracking)...take your pick depending on how much you struggle with a particular fight.
Boss mods and a good raid UI help your performance a lot, but do not give you any post-match analysis.
I strongly recommended getting onto www.worldoflogs.com and live logging when you raid. If you are new to this you'll be initially overwhelmed with the amount of information it provides, so for the first few times just think of it as a dps/hps meter. As you play with it you'll find all sorts of useful information. e.g. noticing your 'best' dps is great on bosses but slow to switch to ads etc.
You can also post links of your raid, when troubleshooting a difficult encounter, and helpful people here or in other places will be able to see what the weak spots are.